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Digitized  by  the  Internet  Archive 

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http://www.archive.org/details/elementarycQurseOOmahaiala 


AN 


ELEMENTARY  COURSE 


OF 


CIVIL    ENGINEERING, 


FOR  THE  USE  OF 


CADETS  OF  THE  UNITED  STATES'  MILITARY  ACADE^I 


Hi 


D.  H.  MAHAN,  M.  A.. 

PROFESSOR  OF  MILITARY  AND  CIVIL  ENGINEERING  IN  THE 
MILITARY  ACADEMY. 


SIXTH  EDITION, 

WITH    LARGE    ADDENDA    AND    MANY   NEW    CUTS. 


NEW  YORK : 
JOHN     WILEY, 

66    WALKER-STREET. 
1861. 


Entered,  a:cording  lo  act  </•  Congress,  in  tho  yeai  LB46,  by 
WILEY     AND    PUTNAM, 
«  Clerk's  Office  of  the  District  Court  for  the  Southern  District  of  New  Tor*. 


PREFACE. 


The  present  Edition  of  this  Work,  like  the  two 
preceding,  has  been  compiled  for  the  use  of  the  ca- 
dets of  the  U.  S.  Military  Academy,  and  comprises 
that  part  of  the  Course  of  Civil  Engineering  taught 
them  which  the  Author  deemed  would  prove  the  most 
useful  to  pupils  in  other  seminaries,  studying  for  the 
profession  of  the  civil  engineer. 

In  preparing  this  Edition,  the  Author  has  found  it 
necessary  to  recast  and  rewrite  the  greater  portion 
of  the  work ;  owing  to  the  considerable  additions 
made  to  it,  and  called  for  by  the  vast  accumulation 
of  important  facts  since  the  publication  of  the  former 
editions.  A  new  form  has  also  been  given  to  the 
work,  in  the  substitution  of  wood-cuts  in  the  bodv 
of  it  for  the  plates  in  the  former  editions,  as  better 
adapted  to  its  main  object  as  a  text-book.  From 
these  additions  and  changes,  the  Author  trusts  that 
the  work  will  be  found  to  contain  all  of  the  essentia] 


PREFACE. 


principles  and  facts  respecting  those  branches  of  the 
subject  of  which  it  treats;  and  that  it  will  prove  a 
serviceable  aid  to  instructors  and  pupils,  in  opening 
the  way  to  a  more  extensive  Drosecution  ot'  the 
studies  connected  witn  the  engineer's  art. 


CONTENTS 


A«T.      pAflk 

BUILDING  MATERIALS. 

Classification  of 3  ..  1 

Stone. 

Classification  of  Stones 3  . .  1 

Silicious  Stones   4  . .  2 

Argillaceous  Stones 17  . .  5 

Calcareous  Stones 21   ..  6 

Durability  of  Stones 31   . .  10 

Hardness  of  Stones 37  . .  12 

Lime. 

Classification  of  Limes 38  . .  14 

Hydraulic  Limes  and  Cements 42  . .  16 

Physical  Characters  and  Tests  of  Hydraulic  Limestones    ....  49  . .  18 

Calcination  of  Limestone    54  . .  21 

Lime-kilns 59  . .  22 

Slaking  Lime 67  . .  26 

Manner  of  reducing  Hydraulic  Cement 78  . .  28 

Artificial  Hydraulic  Limes  and  Cements 81   . .  29 

Puzzolana,  &c 87  . .  30 

Mortar. 

Classification  of  Mortars 96  . .  32 

Sand 100  . .  33 

Hydraulic  Mortar 107  . .  34 

Mortars  exposed  to  Weather 113  ..  35 

Manipulations  of  Mortar  116  . .  36 

Setting  and  Durability  of  Mortar 119  . .  37 

Concrete    128  . .  39 

Beton 133  . .  4C 

Adherence  of  Mortar 137  . .  ib. 

Mastics. 

Bituminous  Mastics 143  . .  41 

Glce. 

Marine  Glue 153  . .  43 


6  CONTENTS. 

Art.    Pabi 

Brick. 

Uses,  &c 155  ..  44 

Fire  Brick 167  ..  46 

Tiles 168  . .  ib. 

"WQnn 

Classification  of  Timber 169  . .  ib. 

Felling  of  Timber 172  . .  47 

Seasoning  of  Timber 175  . .  48 

Defects  of  Timber 181  . .  49 

Preservation  of  Timber 184  . .  5C 

Durability  of  Timber 194  . .  52 

Trees  which  furnish  Timber 198  .  .  53 

Metals. 

Cast  Iron 206  . .   55 

Wrought  Iron 220  . .   58 

Durability  of  Iron 234  . .   59 

Preservation  of  Iron 246  .     60 

Copper 255  . .   63 

Zinc 256  . .  ib. 

Tin 258  . .   64 

Lead 259  . .  ib. 

Paints  and  Varnishes. 

Compositions  of 265  . .   65 

Varnish  and  Paint  for  Zincked  Iron 267  . .   66 

Results  of  Experimental  Researches  on  the  Strength  of 
Materials. 

General  Remarks  on 270  .  .   67 

Strength  of  Stone 281  .  .   70 

Practical  Deductions  on  the  Strength  of  Stone 288  . .   74 

Expansion  of  Stone  by  Heat 289  . .    75 

Strength  of  Mortars 290  .  .   75 

Strength  of  Concrete  and  Beton 295  .  .    77 

Strength  of  Timber 296  . .   78 

Strength  of  Cast  Iron 303  . .   81 

Strength  of  Wrought  Iron 324  .  .   96 

Resistance  to  Torsion  of  Wrought  and  Cast  Iron 333  .  .  102 

Strength  of  Copper 334  .  .  103 

Strength  of  other  Metals 336  .  .  104 

Linear  Dilatation  of  Metals  by  Heat 337  . .  ib. 

Adhesion  of  Iron  Spikes  to  Timber 338  .  .  105 


MASONRY. 

Classification  of  Masonry 344  . .  107 

Cut  Stone 345  .  .  ib. 

Rubble  Stone 358  .  .  113 


CONTENTS.  7 

Art.  Pag* 

BricA  Masonry 362  .  113 

Foundations 366  . .  114 

Precautions  against  Lateral  Yielding  in  Foundations 393  . .  124 

Foundations  in  Water 394  . .  127 

Construction  of  Foundation  Courses 406  . .  136 

Component  Parts  of  Structures  of  Masonry 412  . .  138 

Walls  of  Enclosures 413  . .  138 

Vertical  Supports 414  . .  139 

Areas 415  ..  ib. 

Retaining  Walls 416  . .  ib. 

Relieving  Arches 429  . .  145 

Lintel 435  . .  146 

Plate-bande 436  . .  ib. 

Arches 437  . .  147 

Precautions  against  Settling ,  479  . .  163 

Pointing 480  . .  164 

Repairs  of  Masonry 483  . .  165 

Effects  of  Temperature ft 486  . .  166 


FRAMING. 

General  Principles  of  Framing 490  . .  168 

Frames  of  Timber j 499  . .  170 

Joints  of  Frames 520  . .  185 

Frames  of  Iron 527   . .  189 

Flexible  Supports  for  Frames 534  . .  196 

Experiments  on  the  Strength  of  Frames 549  . .  200 


BRIDGES. 

Classification  of  Bridges 550  . .  204 

Stone  Bridges 551   .  .  ib. 

Wooden  Bridges 583   . .  230 

Cast-iron  Bridges 606   .  .  244 

Effects  of  Temperature  on  Stone  and  Cast-iron  Bridges  ....  611   . .  250 

Suspension  Bridges 612   . .  251 

Moveable  Bridges 624   . .  264 

Aqueduct  Bridges 630  . .  269 


ROADS. 

Reconnaissance    635  . .  277 

Survey 636   . .  278 

Map  and  Memoir 637  . .  279 

Location  of  Common  Roads ■. 638   . .     ib. 

Earth-work 646  . .  284 

Drainage 653 

Road-coverings 654  . .  292 

Pavements 655  . .     ib. 

Broken-stone  Road-covering 656  . .  296 

Materials  and  Repairs 661   .     298 

Cross  Dimensions  of  Roads 662  .     299 


CONTENTS. 


RAILWAYS. 

Art.        Pa  si 

Rails 665  . .   301 

Supports 668  . .   303 

Chairs 669  . .  304 

Ballast 670  ..     ib. 

Railways  of  WooJ  and  Iron 671  . .  305 

Gauge 672  . .     ib. 

Curves 674  . .   306 

Sidings,  &c 675  . .   307 

Turn-plates 676  . .  308 

Street-crossings 677  . .     ib. 

Gradients 678  . .     ib. 

Tunnels.... 680  ..   309 

Masonry  of 'Tunnels 682  . .  311 


CANALS. 

Classification  of  Canals 687  . .  313 

Level  Canals 689  . .  ib. 

Canals  not  on  the  same  Level 690  . .  315 

Feeders  and  Reservoirs 697  . .  321 

Lift  of  Locks 704  . .  326 

Levels 705  . .  327 

Locks 706  . .  ib. 

Lock-gates 722  . .  333 

Accessory  Works ;. .  726  . .  335 

General  Dimensions  of  Canals 733  . .  337 


RIVERS. 

Natural  Features  of  Rivers 734  . .  340 

River  Improvements 740  . .  341 

Means  for  protecting  River-banks 742  . .  342 

Measures  against  Inundations 743  . .  ib. 

Elbows 744  . .  343 

Bars 747  . .  344 

Slack- water  Navigation 752  . .  346 


SEA-COAST  IMPROVEMENTS. 

Classification  of,  &c 760  . .  349 

Roadsteads 765  . .  350 

Harbors 766  ..  353 

Dikes 777  ..  356 

Groins 778  ..  357 

tea-walls 779  ..  358 


ELEMENTARY  COURSE 


CIVIL  ENGINEERING. 


BUILDING  MATERIALS. 

1 .  A  knowledge  of  the  properties  of  building  materials  rs  one 
of  the  most  important  branches  of  Civil  Engineering.  An  en- 
gineer, to  be  enabled  to  make  a  judicious  selection  of  materials, 
and  to  apply  them  so  that  the  ends  of  sound  economy  and  skilful 
workmanship  shall  be  equally  subserved,  must  know  their  or- 
dinary durability  under  the  various  circumstances  in  which  they 
are  employed,  and  the  means  of  increasing  it  when  desirable  ; 
their  capacity  to  sustain,  without  injury  to  their  physical  quali- 
ties, permanent  strains,  whether  exerted  to  crush  them,  tear  them 
asunder,  or  to  break  them  transversely ;  their  resistance  to  rup- 
ture and  wear,  from  percussion  and  attrition ;  and,  finally,  the 
time  and  expense  necessary  to  convert  them  to  the  uses  for  which 
they  may  be  required. 

2.  The  materials  in  general  use  for  civil  constructions  may  be 
arranged  under  the  three  following  heads  : 

1st.  Those  which  constitute  the  more  solid  components  of 
structures,  as  Stone,  Brick,  Wood,  and  the  Metals. 

2d.  The  cements  in  general,  as  Mortar,  Mastics,  Glue,  &c, 
which  are  used  to  unite  the  more  solid  parts. 

3d.  The  various  mixtures  and  chemical  preparations,  as  solu- 
tions of  Salts,  Paints,  Bituminous  Substances,  &c,  employed 
to  coat  the  more  solid  parts,  and  protect  them  from  the  chemical 
and  mechanical  action  of  atmospheric  changes,  and  other  causes 
of  destructibility. 

STONE. 

3  The  term  Stone,  or  Rock,  is  applied  to  any  aggregation  of 
several  mineral  substances.  Stones,  for  the  convenience  of  de- 
scription, may  be  arranged  under  three  general  heads — the  sili- 
cious,  the  argillaceous,  and  the  calcareous. 

1 


2  BUILDING  MATERIALS. 

4.  Silicious  Stones.  The  stones  arranged  under  this  head 
receive  their  appellation  from  silex,  the  principal  constituent  of  the 
minerals  whxh  compose  them.  They  are  also  frequently  desig- 
nated, either  according  to  the  mineral  found  most  abundantly  in 
them,  or  from  the  appearance  of  the  stone,  a?  feldspathic,  quart- 
zose,  arenaceous,  &c. 

5.  The  silicious  stones  generally  do  not  effervesce  with  acids, 
and  emit  sparks  when  struck  with  a  steel.  They  possess,  in  a 
high  degree,  the  properties  of  strength,  hardness,  and  durability  ; 
and,  although  presenting  great  diversity  in  the  degree  of  these 
properties,  as  well  as  in  their  structure,  they  furnish  an  <  xtensive 
variety  of  the  best  stone  for  the  various  purposes  of  the  engineer 
and  architect. 

6.  Sienite,  Porphyry,  and  Green-stone,  from  the  abundance 
of  feldspar  which  they  contain,  are  often  designated  as  feldspathic 
rocks.  For  durability,  strength,  and  hardness,  they  may  be  placed 
in  the  first  rank  of  silicious  stones. 

7.  Sienite  consists  of  a  granular  aggregation  of  feldspar,  horn- 
blende, and  quartz.  It  furnishes  one  of  the  most  valuable  building 
stones,  particularly  for  structures  which  require  great  strength, 
or  are  exposed  to  any  very  active  causes  of  destructibility,  as 
sea  walls,  lighthouses,  and  fortifications.  Sienite  occurs  in  exten- 
sive beds,  and  may  be  obtained,  from  the  localities  where  it  is 
quarried,  in  blocks  of  any  requisite  size.  It  does  not  yield  easily 
to  the  chisel,  owing  to  its  great  hardness,  and  when  coarse- 
grained it  cannot  be  wrought  to  a  smooth  surface.  Like  all 
stones  in  which  feldspar  is  found,  the  durability  of  sienite  de- 
pends essentially  upon  the  composition  of  this  mineral,  which, 
owing  to  the  potash  it  contains,  sometimes  decomposes  very  rap- 
idly when  exposed  to  the  weather.  The  durability  of  feldspathic 
rocks,  however,  is  very  variable,  even  where  their  composition  is 
the  same  ;  no  pains  should  therefore  be  spared  to  ascertain  this 
property  in  stone  taken  from  new  quarries,  before  using  it  for 
important  public  works. 

8.  Porphyry.  This  stone  is  usually  composed  of  compact  feld- 
spar, having  crystals  of  the  same,  and  sometimes  those  of  othei 
minerals,  scattered  through  the  mass.  Porphyry  furnishes  stones 
of  various  colors  and  texture  ;  the  usual  color  being  reddish,  ap 
proaching  to  purple,  from  which  the  stone  takes  its  name.  One 
of  the  most  beautiful  varieties  is  a  brecciated  porphyry,  consist 
ing  of  angular  fragments  of  the  stone  united  by  a  cement  of  com 
pact  feldspar.  Porphyry,  from  its  rareness  and  extreme  hardness 
is  seldom  applied  to  any  other  than  ornamental  purposes.  Th( 
best  known  localities  of  sienite  and  porphyry  are  in  the  neighbor 
hood  of  Boston. 

9.  Green-stone.     This  stone  is  a  mixture  of  hornblende  wiu 


STONE. 


common  and  compact  feldspar,  presenting  sometimes  a  granular 
though  usually  a  compact  texture.  Its  ordinary  color,  when  dry 
is  some  shade  of  brown ;  but,  when  wet,  it  becomes  green  "sh, 
from  which  it  takes  its  name.  Green-stone  is  very  hard,  and 
one  of  the  most  dm  able  rocks ;  but,  occurring  in  small  and 
iiregular  blocks,  its  uses  as  a  building  stone  are  very  restricted. 
When  walls  of  this  stone  are  built  with  very  white  mortar,  they 
present  a  picturesque  appearance,  and  it  is  on  that  account  well 
adapted  to  rural  architecture.  Green-stone  might  also  be  used 
as  a  material  for  road-making ;  large  quantities  of  it  are  annually 
taken  from  the  principal  locality  of  this  rock  in  the  United  States, 
so  well  known  as  the  Palisades,  on  the  Hudson,  for  construct- 
ing wharves,  as  it  is  found  to  withstand  jvell  the  action  of  aalt 
water. 

10.  Granite  and  Gneiss.  The  constituents  of  these  two  stones 
are  the  same ;  being  a  granular  aggregation  of  quartz,  feldspar, 
and  mica,  in  variable  proportions.  They  differ  only  in  their 
structure ;  gneiss  being  a  stratified  rock,  the  ingredients  of 
which  occur  frequently  in  a  more  or  less  laminated  state.  Gneiss, 
although  less  valuable  than  granite,  owing  to  the  effect  of  »ts 
structure  on  the  size  of  the  blocks  which  it  yields,  and  from  Jts 
not  splitting  as  smoothly  as  granite  across  its  beds  of  stratifica- 
tion, furnishes  a  building  stone  suitable  for  most  architectural 
purposes.  It  is  also  a  good  flagging  material,  when  it  can  be  ob- 
tained in  thin  slabs. 

Granite  varies  greatly  in  quality,  according  to  its  texture  and 
the  relative  proportions  of  its  constituents.  When  the  quartz  is 
in  excess,  it  renders  the  stone  hard  and  brittle,  and  very  difficult 
to  be  worked  with  the  chisel.  An  excess  of  mica  usually  make? 
the  stone  friable.  An  excess  of  feldspar  gives  the  stone  a  white 
hue,  and  makes  it  freer  under  the  chisel.  The  best  granites  are 
those  with  a  fine  grain,  in  which  the  constituents  seem  uniformly 
disseminated  through  the  mass.  The  color  of  granite  is  usually 
some  shade  of  gray  ;  when  it  varies  from  this,  it  is  owing  to  the 
color  of  the  feldspar.  One  of  its  varieties,  known  as  Oriental 
granite,  has  a  fine  reddish  hue,  and  is  chiefly  used  for  ornamental 
purposes.  Granite  is  sometimes  mistaken  for  sienite,  when  il 
contains  but  little  mica. 

The  quality  of  granite  is  affected  by  the  foreign  minerals  which 
it  may  contain  ;  hornblende  is  said  to  render  it  tough,  and  schor) 
makes  it  quite  brittle.  The  protoxide  and  sulphurets  of  iron  are 
the  most  injurious  in  their  effects  on  granite  ;  the  former  by  con 
version  into  a  peroxide,  and  the  latter  by  decomposing,  destroying 
the  structure  of  the  stone,  and  causing  it  to  break  up  and  disin- 
tegrate. 

Granite,  gneiss,  and  sienite,  differ  so  little  in  their  essential 


BUILDING  MATERIALS. 


qualilies,  as  a  building  material,  that  they  may  be  used  iruliffer 
ently  for  all  structures  of  a  solid  and  durable  character.  They 
ore  extensively  quarried  in  most  of  the  New  England  States,  in 
New  York,  and  in  some  of  the  other  Stales  intersected  by  the 
great  range  of  primitive  rocks,  where  the  quarries  "ie  contiguous 
to  tide-water. 

1 1 .  Mica  Slate.  The  constituents  of  this  stone  are  quartz  and 
mica ;  the  latter  predominating.  It  is  principally  used  as  a  flag- 
ging stone,  and  as  ajire  stone,  or  lining  for  furnaces. 

1 2.  Buhr,  or  Millstone.  This  is  a  very  hard,  durable  stone, 
presenting  a  peculiar,  honeycomb  appearance.  It  makes  a  good 
building  material  for  common  purposes,  and  is  also  suitable  foi 
roaJ  coverings. 

13.  Horn-stone.  This  is  a  highly  silicious  and  very  hard 
stone.  It  resembles  flint  in  its  structure,  and  takes  its  name 
from  its  translucent,  horn-like  appearance.  It  furnishes  a  very 
good  road  material. 

14.  Steatite,  or  Soap-stone.  This  stone  is  a  partially  indura- 
ted talc.  It  is  a  very  soft  stone,  and  not  suitable  for  ordinary 
building  purposes.  It  furnishes  a  good  fire-stone,  and  is  used 
for  the  lining  of  fireplaces. 

15.  Talcose  Slate.  This  stone  resembles  mica  slate,  being  an 
aggregation  of  quartz  and  talc.  It  is  applied  to  the  same  pur- 
poses as  mica  slate. 

16.  Sand-stone.  This  stone  consists  of  grains  of  silicious 
sand,  arising  from  the  disintegration  of  silicious  rocks,  which  are 
united  by  some  natural  cement,  generally  of  an  argillaceous  or  a 
silicious  character. 

The  strength,  hardness,  and  durability  of  sand-stone  vary  be- 
tween very  wide  limits.  Some  varieties  being  little  inferior  to 
good  granite,  as  a  building  stone,  others  being  very  soft,  friable, 
and  disintegrating  rapidly  when  exposed  to  the  weather.  The 
least  durable  sand-stones  are  those  which  contain  the  most  argil- 
laceous matter ;  those  of  a  feldspathic  character  are  also  found 
not  to  withstand  well  the  action  of  weather. 

Sand-stone  is  used  very  extensively  as  a  building  stone,  for 
flagging,  for  road  materials,  and  some  of  its  varieties  furnish  an 
excellent  fire-stone.  Most  of  the  varieties  of  sand-stone  yield 
readily  under  the  chisel  and  saw,  and  split  evenly,  and,  from 
these  properties,  have  received  from  workmen  the  name  of  free 
stone.  The  colors  of  sand-stone  present  also  a  variety  of  shades, 
principally  of  gray,  brown,  and  red. 

The  formations  of  sand-stone  in  the  United  States  are  very 
extensive,  and  a  number  of  quarries  are  worked  in  New  England., 
New  York,  and  the  Middle  States.  These  formations,  and  the 
character  of  the  stone  obtained  from  them,  are  minutely  described 


STONE.  5 

in  the  Geological  Reports  of  these  States,  which  have  been  pub- 
lished within  the  last  few  years. 

Most  of  the  stone  used  for  the  public  buildings  in  Washington, 
is  a  sand-stone  obtained  from  quarries  on  Acquia  Creek  and  the 
Rappahannock.  Much  of  this  stone  is  feldspathic,  possesses  but 
little  strength,  and  disintegrates  rapidly.  The  red  sand-stones 
which  are  used  in  our  large  cities,  are  either  from  quarries  in  a 
formation  extending  from  the  Hidson  to  North  Carolina,  or  from 
a  separate  deposite  in  the  valley  of  the  Connecticut.  The  most 
durable  and  hard  portions  of  these  formations  occur  in  the  neigh 
borhood  of  trap  dikes.  The  fine  flagging-stone  used  in  our  cities 
is  mostly  obtained,  either  from  the  Connecticut  quarries,  or  from 
others  near  the  Hudson,  in  the  Catskill  group  of  mountains. 
Many  quarries,  which  yield  an  excellent  building  stone,  are 
worked  in  the  extensive  formations  along  the  Appalachian  range, 
which  extends  through  the  interior,  through  New  York  and  Vir- 
ginia, and  the  intermediate  States. 

17.  Argillaceous  Stones.  The  stones  arranged  under  this 
head  are  mostly  composed  of  clay,  in  a  more  or  less  indurated 
state,  and  presenting  a  laminated  structure.  They  vary  greatly 
m  strength,  and  are  generally  not  durable,  decomposing  in  some 
cases  very  rapidly,  from  changes  in  the  metallic  sulphurets  and 
salts  found  in  most  of  them.  The  uses  of  this  class  of  stones 
are  restricted  to  roofing  and  flagging. 

18.  Roofing  Slate.  This  well-known  stone  is  obtained  from 
a  hard,  indurated  clay,  the  surfaces  of  the  lamina  having  a  natu- 
ral polish.  The  best  kinds  split  into  thin,  uniform,  light  slabs ; 
are  free  from  sulphurets  of  iron ;  give  a  clear  ringing  sound  when 
struck ;  and  absorb  but  little  water.  Much  of  the  roofing  slate 
quarried  in  the  United  States  is  of  a  very  inferior  quality,  and 
becomes  rotten,  or  decomposes,  after  a  few  years'  exposure.  The 
durability  of  the  best  European  slate  is  about  one  hundred  years  ; 
and  it  is  stated  that  the  material  obtained  from  some  of  the  quar- 
ries worked  in  the  United  States,  is  not  apparently  inferior  to  the 
best  foreign  slate  brought  into  our  markets.  Several  quarries  of 
roofing  slate  are  worked  in  the  New  England  States,  New  York, 
and  Pennsylvania. 

19.  Grayioacke  Slate.  The  composition  of  this  stone  is 
mostly  indurated  clay.  It  has  a  more  earthy  appearance  than 
argillaceous  slate,  and  is  generally  distinctly  arenaceous.  Its 
colors  are  usually  dark  gray,  or  red.  It  is  quarried  principally 
for  flagging-stone. 

20.  Hornblende  Slate  This  stone,  known  also  as  green-stone 
slate,  properly  belongs  to  the  silicious  class.  It  consists  mostly 
of  hornblende  having  a  laminated  structure.  It  s  chiefly  quarried 
for  flagging-stone. 


6  BUILDING  MATERIALS. 

21.  Calcareous  Stones.  Lime  is  the  p:ncipal  constituent 
of  this  class,  the  carbonates  of  which,  known  as  lime-stone  and 
marble,  furnish  a  large  amount  of  ordinary  b  lilding  stone,  most 
of  the  ornamental  stones,  and  the  chief  ingredient  in  the  compo- 
sition of  the  cements  and  mortars,  used  in  stone  and  brick-work. 
Lime-stone  effervesces  copiously  with  acids ;  its  texture  is  de- 
stroyed by  a  strong  heat,  which  also  drives  off  its  carbonic  acid 
and  water,  converting  it  into  quick  lime.  By  absorbing  water, 
quick-lime  is  converted  into  a  hydrate,  or  slaked  lime  ;  consider- 
able heat  is  evolved  during  this  chemical  change,  and  the  stone 
increases  in  bulk,  and  gradually  crumbles  down  into  a  fine 
powder. 

The  lime-stones  present  great  diversity  in  their  physical  prop- 
erties. Some  of  them  seem  as  durable  as  the  best  silicious  stones, 
and  are  but  little  inferior  to  them  in  strength  and  hardness  ;  others 
decompose  rapidly  on  exposure  to  the  weather ;  and  some  kinds 
are  so  soft  that,  when  first  quarried,  they  can  be  scratched  with 
the  nail,  and  broken  between  the  fingers.  The  lime-stones  are 
generally  impure  carbonates ;  and  we  are  indebted  to  these  im- 
purities for  some  of  the  most  beautiful,  as  well  as  the  most  inval- 
uable materials  used  for  constructions.  Those  which  are  colored 
by  metallic  oxides,  or  by  the  presence  of  other  minerals,  furnish 
the  large  number  of  colored  and  variegated  marbles  ;  while  those 
which  contain  a  certain  proportion  of  clay,  or  of  magnesia,  yield, 
on  calcination,  those  cements  which,  from  their  possessing  the 
property  of  hardening  under  water,  have  received  the  various 
appellations  of  hydraulic  lime,  water  lime,  Ro?nan  cement,  &c. 

Lime-stone  is  divided  into  two  principal  classes,  granular 
lime-stone  and  compact  lime-stone.  Each  of  these  furnishes  both 
the  marbles  and  ordinary  building  stone.  The  varieties  not  sus- 
ceptible of  receiving  a  polish,  are  sometimes  called  common  lime- 
stone. 

The  granular  lime-stones  are  generally  superior  to  the  compact 
for  building  purposes.  Those  which  have  the  finest  grain  are  the 
best,  both  for  marbles  and  ordinary  building  stone.  The  coarse- 
grained varieties  are  frequently  friable,  and  disintegrate  rapidly 
when  exposed  to  the  weather.  All  the  varieties,  both  of  the  com- 
pact and  granular,  work  freely  under  the  chisel  and  grit-saw,  and 
may  be  obtained  in  blocks  of  any  suitable  dimensions  for  the 
heaviest  structures. 

The  durability  of  lime-stone  is  very  materially  affected  by  the 
foreign  minerals  it  may  contain  ;  the  presence  of  clay  injures  the 
stone,  particularly  when,  as  sometimes  happens,  it  runs  through 
the  bed  in  very  minute  veins  :  blocks  of  stone  having  this  imper- 
fection, soon  separate  along  these  veins  on  exposure  to  moisture. 
The  protoxide,  the  protocarbonate,  and  the  sulphuret  of  iron,  are 


STONE. 


also  very  destructive  in  their  effects  ;  frequently  causing,  by  their 
chemical  changes,  rapid  disintegration. 

Among  the  varieties  of  impure  carbonates  of  lime,  the  magne- 
sian  lime-stones,  called  dolomites,  merit  to  be  particularly  no- 
ticed. They  are  regarded  in  Europe  as  a  superior  building 
material ;  those  being  considered  the  best  which  are  most  crys- 
talline, and  are  composed  of  nearly  equal  proportions  of  the 
carbonates  of  lime  and  magnesia.  Some  of  the  quarries  of  this 
stone,  which  have  been  opened  in  New  York  and  Massachusetts, 
have  given  a  different  result ;  the  stone  obtained  from  them  being, 
in  some  cases,  extremely  friable. 

22.  Marbles.  The  term  marble  is  now  applied  exclusively 
to  any  lime-stone  which  will  receive  a  polish.  Owing  to  the  cost 
of  preparing  marble,  it  is  restricted  in  its  uses  to  ornamental  pur- 
poses. The  marbles  present  great  variety,  both  in  color  and  ap- 
pearance, and  have  generally  received  some  appropriate  name 
descriptive  of  these  accidents. 

23.  Statuary  Marble  is  of  the  purest  white,  finest,  grain,  and 
free  from  all  foreign  minerals.  It  receives  that  delicate  polish, 
without  glare,  which  admirably  adapts  it  to  the  purposes  of  the 
sculptor,  for  whose  uses  it  is  mostly  reserved. 

24.  Conglomerate  Marble.  This  consists  of  two  varieties  ;  the 
one  termed  pudding  stone,  which  is  composed  of  rounded  pebbles 
imbedded  in  compact  lime-stone  ;  the  other  termed  breccia,  con- 
sisting of  angular  fragments  united  in  a  similar  manner.  The 
colors  of  these  marbles  are  generally  variegated,  forming  a  very 
handsome  ornamental  material. 

25.  Birds-eye  Marble.  The  name  of  this  stone  is  descriptive 
of  its  appearance,  which  arises  from  the  cross  sections  of  a  pecu- 
liar fossil  (fucoides  demissus)  contained  in  the  mass,  made  in 
sawing  or  splitting  it. 

26.  Lumachella  Marble.  This  is  obtained  from  a  lime-stone 
having  shells  imbedded  in  it,  and  takes  its  name  from  this  cir- 
cumstance. 

27.  Verd  Antique.  This  is  a  rare  and  costly  variety,  of  a 
beautiful  green  color,  caused  by  veins  and  blotches  of  serpentine 
diffused  through  the  lime-stone. 

28.  The  terms  veined,  golden,  Italian,  Irish,  &c,  given  to 
the  marbles  found  in  our  markets,  are  significant  of  their  appear- 
ance, or  of  the  localities  from  which  they  are  procured 

29.  Lime-stone  is  so  extensively  diffused  throughout  the  Uni- 
ted States,  and  is  quarried,  either  for  building  stone  or  to  furnish 
lime,  in  so  many  localities,  that  it  would  be  impracticable  to  enu 
merate  all  within  any  moderate  compass.  One  of  the  most  re- 
markable formations  of  this  stone  extends,  in  an  uninterrupted 
oed,  from  Canada,  through  the  States  of  Vermont,  Mass.,  Conn., 


8  BUILDING  MATERIALS. 

New  York,  New  Jersey,  Penn.,  and  Virg.,  and,  in  all  probability 
much  farther  south. 

Marbles  are  quarried  in  various  localities  in  the  United  States 
Among  the  most  noted  are  the  quarries  in  Berkshire  Co.,  Mass., 
which  furnish  both  pure  and  variegated  marbles ;  those  on  the 
Potomac,  from  which  the  columns  of  conglomerate  marbles  were 
obtained  that  are  seen  in  the  interior  of  the  Capitol  at  Washing- 
ton ;  several  in  New  York,  which  furnish  white,  the  birds-eye,  and 
other  variegated  kinds ;  and  some  in  Conn.,  which,  among  other 
varieties,  furnish  a  verd  antique  of  handsome  quality. 

Lime-stone  is  burned,  either  for  building  or  agricultural  pur- 
poses, in  almost  every  locality  where  deposites  of  the  stone  occur. 
Thomaston,  in  Maine,  has  supplied  for  some  years  most  of  the 
markets  on  the  sea-board  with  a  material  which  is  considered  as 
a  superior  article  for  ordinary  building  purposes.  One  of  the 
greatest  additions  to  the  building  resources  of  our  country,  was 
made  in  the  discovery  of  the  hydraulic  or  water  lime-stones  of 
New  York.  The  preparation  of  this  material,  so  indispensable 
for  all  hydraulic  works  and  heavy  structures  of  stone,  is  carried 
on  extensively  at  Roundout,  on  the  Delaware  and  Hudson  canal, 
in  Madison  Co.,  and  is  sent  to  every  part  of  the  United  States, 
being  in  great  demand  for  all  the  public  works  carried  on  under 
the  superintendence  of  our  civil  and  military  engineers.  A  not 
less  valuable  addition  to  our  building  materials  has  been  made  by 
Prof.  W.  B.  Rogers,  who,  a1  few  years  since,  directed  the  atten- 
tion of  engineers  to  the  dolomites,  for  their  good  hydraulic  prop- 
3rties.  From  experiments  made  by  Vicat,  in  France,  who  first 
there  observed  the  same  properties  in  the  dolomite,  and  from 
those  in  our  own  country,  it  appears  highly  probable  that  the  mag- 
nesian  lime-stones,  containing  a  certain  proportion  of  magnesia, 
will  be  found  fully  equal  to  the  argillaceous,  from  which  hydraulic 
lime  has  hitherto  been  solely  obtained. 

Both  of  these  lime-stones  belong  to  very  extensive  formations. 
The  hydraulic  lime-stones  of  New  York  occur  in  a  deposite  called 
the  Water-lime  Group,  in  the  Geological  Survey  of  New  York 
corresponding  to  formation  VI.  of  Prof.  H.  B.  Rogers'  arrange- 
ment of  the  rocks  of  Penn.  This  formation  is  co-extensive  with 
the  Helderberg  Range  as  it  crosses  New  York ;  it  is  exposed  in 
many  of  the  valleys  of  Penn.  and  Virg.,  west  of  the  Great  Valley. 
It  may  be  sought  for  just  below  or  not  far  beneath  the  Oriskan) 
sand-stones  of  the  New  York  Survey,  which  correspond  to  form- 
ation VII.  of  Rogers.  This  sand-stone  is  easily  recognised,  being 
of  a  yellowish  white  color,  gianular  texture,  with  large  cavities 
left  by  decayed  shells.  The  lime-stone  is  usually  an  earthy 
drab-colored  rock,  sometimes  a  greenish  blue,  which  does  no 
ilake  after  being  burned. 


STONE. 


The  hydraulic  magnesian  lime-stones  belong  to  the  formations 
II.  and  Vl.  of  Rogers  ;  the  first  of  these  is  the  same  as  the  Black 
River,  or  Mohawk  lime-stone  of  the  New  York  Survey.  It  is  the 
oldest  fossiliferous  lime-stone  in  the  United  States,  and  occura 
throughout  the  whole  bed,  associated  with  the  slates  which  occu- 
py formation  III.  of  Rogers,  and  are  called  the  Hudson  River 
Group  in  the  New  York  Survey.  This  extensive  bed  lies  in  the 
great  Appalachian  Valley,  known  as  the  Valley  of  Lake  Cham- 
plain,  Valley  of  the  Hudson,  as  far  as  the  Highlands,  Cumberland 
Valley,  Valley  of  Virginia,  and  Valley  of  East  Tennessee.  The 
same  stone  is  found  in  the  deposites  of  some  of  the  western  val- 
leys of  the  mountain  region  of  Penn.  and  Virginia. 

The  importance  of  hydraulic  lime  to  the  security  of  structures 
exposed  to  constant  moisture,  renders  a  knowledge  of  the  geo- 
logical positions  of  those  lime-stones  from  which  it  can  be  ob 
tained  an  object  of  great  interest.  From  the  results  of  the  various 
geological  surveys  made  in  the  United  States,  and  in  Europe, 
lime-stone,  possessing  hydraulic  properties  when  calcined,  may 
be  looked  for  among  those  beds  which  are  found  in  connection 
with  the  shales,  or  other  argillaceous  deposites.  The  celebrated 
Roman,  or  Parker's  cement,  of  England,  which,  from  its  prompt 
induration  in  water,  has  become  an  important  article  of  commerce, 
is  manufactured  from  nodules  of  a  concretionary  argillaceous 
lime-stone,  called  septaria,  from  being  traversed  by  veins  of 
sparry  carbonate  of  lime.  Nodules  of  this  character  are  found 
in  Mass.,  and  in  some  other  States  ;  and  it  is  probable  they  would 
yield,  if  suitably  calcined  and  ground,  an  article  in  nowise  inferior 
to  that  imported. 

30.  Gypsum,  or  Plaster  of  Paris.  This  stone  is  a  sulphate  of 
lime,  and  has  received  its  name  from  the  extensive  use  made  of 
it  at  Paris,  and  in  its  neighborhood,  where  it  is  quarried  and  sent  to 
all  parts  of  the  world;  being  of  a  superior  quality,  owing,  it  is  stated, 
to  a  certain  portion  of  carbonate  of  lime  which  the  stone  contains. 
Gypsum  is  a  very  soft  stone,  and  is  not  used  as  a  building  stone. 
Its  chief  utility  is  in  furnishing  a  beautiful  material  for  the  orna- 
mental casts  and  mouldings  in  the  interior  of  edifices.  For  this 
purpose  it  is  prepared  by  calcining,  or,  as  the  workmen  term  it, 
boiling  the  stone,  until  it  is  deprived  of  its  water  of  crystallization. 
In  this  state  it  is  made  into  a  thin  paste,  and  poured  into  moulds  to 
form  the  cast,  in  which  it  hardens  very  promptly.  Ca  Icined  plaster 
of  Paris  is  also  used  as  a  cement  for  stone ;  but  it  is  eminently  unfit 
for  this  purpose  ;  for  when  exposed,  in  any  situation,  to  moisture 
it  absorbs  it  with  avidity,  swells,  cracks,  and  exfoliates  rapidly. 

Gypsum  is  found  in  various  locahties  in  the  United  States 
Large  quantities  of  it  are  quarried  in  New  York,  both  for  build 
uig  and  agricultural  purposes. 

2 


10  BUILDING  MATERIALS. 

31.  Durability  of  Stone.     The  most  important  properties 
of  stone,  as  a  building  material,  are  its  durability  ander  the  or 
dinary  circumstances  of  exposure  to  weather  ;    its  capacity  to 
sustain  high  degrees  of  temperature ;  and  its  resistance  to  the 
destructive  action  of  fresh  and  salt  water. 

The  wear  of  stone  from  ordinary  exposure  is  very  variable, 
depending,  not  only  upon  the  texture  and  constituent  elements  of 
the  stone,  but  also  upon  the  locality  and  the  position  it  may  oc- 
cupy in  a  structure,  with  respect  to  the  prevailing  driving  rains. 
The  chemist  and  geologist  have  not,  thus  far,  laid  down  any  in- 
fallible rules  to  guide  the  engineer  in  the  selection  of  a  material 
that  may  be  pronounced  durable  for  the  ordinary  period  allotted 
to  the  works  of  man.  In  truth,  the  subject  admits  of  only  gen- 
eral indications  ;  for  stones  having  the  same  texture  and  chemical 
composition,  from  causes  not  fully  ascertained,  are  found  to  pos- 
sess very  different  degrees  of  duration.  This  has  been  particu- 
larly noted  in  feldspathic  rocks.  As  a  general  rule,  those  stones 
which  are  fine-grained,  absorb  least  water,  and  are  of  greatest 
specific  gravity,  are  also  most  durable  under  ordinary  exposures. 
The  weight  of  a  stone,  however,  may  arise  from  a  large  propor- 
tion of  iron  in  the  state  of  a  protoxide,  a  circumstance  generally 
unfavorable  to  its  durability.  Besides,  the  various  chemical  com 
binations  of  iron,  potash  and  clay,  when  found  in  considerable 
quantities,  both  in  the  primary  and  sedimentary  silicious  rocks, 
greatly  affect  their  durability.  The  potash  contained  in  feldspar 
dissolves,  and  carrying  off  a  considerable  proportion  of  the  sjiica, 
leaves  nothing  but  aluminous  matter  behind.  The  clay,  on  the 
other  hand,  absorbs  water,  becomes  soft,  and  causes  the  uLome  to 
crumble  to  pieces.  Iron  in  the  form  of  protoxide,  in  sonv^  cases 
only,  discolors  the  stone  by  its  conversion  into  a  peroxide.  This 
discoloration,  while  it  greatly  diminishes  the  value  of  some  .'.'.ones, 
as  in  white  marble,  in  others  is  not  disagreeable  to  the  eye,  pro 
ducing  often  a  mottled  appearance  in  buildings  which  ?.ad;:  to  the 
picturesque  effect. 

32.  Frost,  or  rather  the  alternate  actions  of  freezing  and  thaw 
ing,  is  the  most  destructive  agent  of  Nature  with  which  the  en 
gineer  has  to  contend.  Its  effects  vary  with  the  textuie  of  stones  ; 
those  of  a  fissile  nature  usually  splitting,  while  the  more  porous 
kinds  disintegrate,  or  exfoliate  at  the  surface.  When  stone  from 
a  new  quarry  is  to  be  tried,  the  best  indication  of  its  resistance  tc 
frost  may  be  obtained  from  an  examination  of  any  rocks  of  the 
same  kind,  within  its  vicinity,  which  are  known  to  have  been 
exposed  for  a  long  period.  Submitting  the  stone  fiesh  from  the 
quarry  to  the  direct  action  of  freezing  would  seem  to  be  the  most 
certain  test,  were  the  stone  destroyed  by  the  expansive  action 

jone  of  frost :  but  besides  the  uncertainty  of  this  test,  it  is  known 


STONE.  11 

that  some  stones,  which,  wnen  first  quarried,  are  much  affected 
by  frost,  splitting  under  its  action,  become  impervious  to  it  afte 
they  have  lost  the  moisture  of  the  quarry,  as  they  do  not  re-absor i 
near  so  large  an  amount  as  they  bring  from  the  quarry. 

33.  M.  Brard,  a  French  chemist,  has  given  a  process  for  as- 
certaining the  effects  of  frost  on  stone,  which  has  met  with  the  ap- 
proval of  many  French  architects  and  engineers  of  standing,  as  ft 
corresponds  with  their  experience.  M.  Brard  directs  that  a  small 
cubical  block,  about  two  inches  on  the  edge,  shaL  \e  carefully 
sawed  from  the  stone  to  be  tested.  A  cold  saturate,  .olution  of 
sulphate  of  soda  is  prepared,  placed  over  a  fire,  and  brought  to 
the  boiling  point.  The  stone,  suspended  from  a  string,  is  im- 
mersed in  the  boiling  liquid,  and  kept  there  during  thirty  minutes  ; 
it  is  then  carefully  withdrawn ;  the  liquid  is  decanted,  free  from 
sediment  into  a  flat  vessel,  and  the  stone  is  suspended  over  it  in 
a  cool  cellar.  An  efflorescence  of  the  salt  soon  makes  its  appear- 
ance on  the  stone,  when  it  must  be  again  dipped  into  the  liquid. 
This  should  be  done  once  or  more  frequently  during  the  day, 
and  the  process  be  continued  in  this  way  for  about  a  week.  The 
earthy  sediment,  found  at  the  end  of  this  period  in  the  vessel,  is 
weighed,  and  its  quantity  will  give  an  indication  of  the  like  effect 
of  frost.  This  process,  with  the  official  statement  of  a  commission 
of  engineers  and  architects,  by  whom  it  was  tested,  is  minutely 
detailed  in  vol.  38,  Annates  de  Cldmie  et  de  Physique,  and  the 
results  are  such  as  to  commend  it  to  the  attention  of  engineers  in 
submitting  new  stones  to  trial. 

34.  By  the  absorption  of  water,  stones  become  softer  and  more 
friable.  The  materials  for  road  coverings  should  be  selected 
from  those  stones  which  absorb  least  water,  and  are  also  hard 
and  not  brittle.  Granite,  and  its  varieties,  lime-stone,  and  com- 
mon sand-stone,  do  not  make  good  road  materials  of  broken  stone. 
All  the  hornblende  rocks,  porphyry,  compact  feldspar,  and  the 
quartzose  rock  associated  with  graywacke,  furnish  good,  durable 
road  coverings.  The  fine-grained  granites  which  contain  but  a 
small  proportion  of  mica,  the  fine-grained  silicious  sand-stones 
which  are  free  from  clay,  and  carbonate  of  lime,  form  a  durable 
material  when  used  in  blocks  for  paving.  Mica  slate,  talcose 
slate,  hornblende  slate,  some  varieties  of  gneiss,  some  varieties 
of  sand-stone  of  a  slaty  structure,  and  graywacke  slate,  yield  ex- 
cellent materials  for  flag-stone. 

35.  The  influence  of  locality  on  *he  durability  of  stone  is  very 
marked.  Stone  is  observed  to  wear  more  rapidly  in  cities  than 
\n  the  country :  and  the  stone  in  those  parts  of  edifices  exposed 
to  the  prevailing  rains  and  winds,  soonest  exhibits  signs  of  decay. 
The  disintegration  of  the  stratified  stones  placed  in  a  wall,  is 
majnlv  affected  by  the  position  which  the  strata    or  quarry  W 


;2 


BUILDING  MVTfRLAU. 


receives,  with  respect  to  the  exposed  S"rface  ;  proceeding  fastei 
when  the  faces  of  the  strata  are  exposed,  than  in  the  contrary 
position. 

3f>.  Stones  which  resist  a  high  degree  of  heat  without  fusing 
are  used  for  lining  furnaces,  and  are  termed  fire-stones.  A  good 
fire -stone  should  not  only  be  infusible,  but  also  not  liable  to  crack 
or  exfoliate  from  heat.  Stones  that  contain  lime,  or  magnesia, 
except  in  the  form  of  silicates,  are  usually  unsuitable  for  fire- 
stones.  Some  porous  silicious  lime-stones,  as  well  as  some  gyp- 
sous silicious  rocks,  resist  moderate  degrees  of  heat.  Stones 
that  contain  much  potash  are  very  fusible  under  high  tempera- 
tures, running  into  a  glassy  substance.  Quartz  and  mica,  in 
various  combinations,  furnish  a  good  fire-stone  ;  as,  for  example, 
finely  granular  quartz  with  thin  layers  of  mica,  mica  slate  of  the 
same  structure,  and  some  kinds  of  gneiss  which  contain  a  large 
proportion  of  arenaceous  quartz.  Several  varieties  of  sand-stone 
make  a  good  lining  for  furnaces.  They  are  usually  those  varie 
ties  which  are  free  from  feldspar,  somewhat  porous,  and  are  un 
crystallized  in  the  mass.  Talcose  slate  likewise  furnishes  a  good 
fire-stone. 

37.  Hardness  is  an  essential  quality  in  stone  exposed  to  wear 
from  the  attrition  of  hard  bodies.  Stones  selected  for  paving,  flag- 
ging, and  steps  for  stairs,  should  be  hard,  and  of  a  grain  suffi 
ciently  coarse  not  to  admit  of  becoming  very  smooth  under  the 
action  to  which  they  are  submitted.  As  great  hardness  adds  to 
the  difficulty  of  working  stone  with  the  chisel,  and  to  the  cost  of 
the  prepared  material,  builders  prefer  the  softer  or  free-stones, 
such  as  the  lime-stones  and  sand-stones,  for  most  building  pur- 
poses. The  following  are  some  of  the  results,  on  this  point,  ob 
tained  from  experiment. 

Table  showing  the  result  of  experiments  made  under  the  direc 
tion  of  Mr.  Walker,  on  the  wear  of  different  stones  in  the  tram- 
way on  the  Commercial  Road,  London,  from  27th  March, 
1830,  to  2\.th  August,  1831,  being  a  period  of  seventeen 
*nonths.     Transactions  of  Civil  Engineers,  vol.  1. 


Name  of  stone. 

Sup.  area 
in  feet. 

Original  weight. 

Loss  of 

weight  by 

wear. 

Loss  per 
sup.  foot. 

Relative 
losses. 

cwt.    qrs. 

lbs. 

Guernsey    . 

4.734 

7      1 

12.75 

4.50 

0.951 

1.000 

Herme  .... 

5.250 

7      3 

24.25 

5.50 

1.048 

1.102 

Budle     .... 

6.336 

9      0 

15.75 

7.75 

1.223 

1.286 

Peterhead  (blue)  . 

3.484 

4      1 

7.50 

6.25 

1.795 

1.887 

Heytor        .     .     . 

4.313 

6      0 

15.25 

8.25 

1.915 

2.014 

Aberdeen  (red) 

5.375 

7      2 

11.50 

11.50 

2.139 

2.249 

Dartmoor    . 

4.500 

6      2 

25.00 

12.50 

2.778 

2.921 

Aberdeen  (blue)  . 

4.823 

6      2 

If  00 

14.75 

3.058 

3.216 

STONE. 


13 


The  v  ">  t\T\ercial  Road  stoneway  consists  of  twc  parallel  lines 
of  rectangu'ar  tramstones,  18  inches  wide  by  12  inches  deep,  and 
jointed  to  eaih  other  endwise,  for  the  wheels  to  travel  on,  with  a 
common  suwvt  pavement  between  for  the  horses. 

The  follow  ii*g  table  gives  the  results  of  some  experiments  on 
the  wear  of  u  line-grained  sand-stone  pavement,  by  M.  Coriolis, 
during  8  ycus,  upon  the  paved  road  from  Paris  to  Toulouse,  the 
carriage  ov-tr  which  is  about  500  tons  daily,  published  in  the 
Annates  des  Ponts  et  Chausees,  for  March  and  April,  1834 


Weight  of  a 
cubic  foot. 

Volume  of  water  absorbed  by  the 
dry  stone  after  one  day's  im- 
mersion, compared   to   that  of 
the  stone. 

Mean  annual 
wear. 

158  lbs. 
154    " 
156    " 
150    " 
148    " 

Neglected  as  insensible. 

M 
H 

T\  in  volume. 

I                       14 

TT 

0.1023  inch. 
0.1063      " 
0.1299      " 
0.2126      " 
0.2677      " 

M.  Coriolis  remarks,  that  the  weight  of  water  absorbed  affords 
one  of  the  best  indications  of  the  durability  of  the  fine-grained 
sand-stones  used  in  France  for  pavements.  An  equally  good  test 
of  the  relative  durability  of  stones  of  the  same  kind,  M.  Coriolis 
states,  is  the  more  or  less  clearness  of  sound  given  out  by  striking 
the  stone  with  a  hammer. 

The  following  results  are  taken  from  an  article  by  Mr.  James 
frost,  Civ.  Engineer,  inserted  in  the  Journal  of  the  Franklin 
Institute  for  Oct.  1835,  on  the  resistance  of  various  substances 
to  abrasion.  The  substances  were  abraded  against  a  piece  of 
white  statuary  marble,  which  was  taken  as  a  standard,  repre- 
sented by  100,  by  means  of  fine  emery  and  sand.  The  relative 
resistance  was  calculated  from  the  weight  lost  by  each  substance 
during  the  operation. 

Comparative  Resistance  to  Abrasion. 


Aberdeen  granite 

Hard  Yorkshire  paving  stone    . 

Italian  black  marble 

980 
327 
260 

Kilkenny  black  marble 

110 

100 
79 

Roman  cement  stone 

69 

Fine-grained  Newcastle  grindstone 

53 
34 

Coarse-grained  Newcastle  grindstone 

i 

14 
19 

14  BUILDING  MATERIALS. 


LIME. 

« 

38.  Lime,  considered  as  a  building  material,  is  now  usually 
divided  into  three  principal  classes  ;  Common,  or  Air  lime,  Hy 
draulic  lime,  and  Hydraulic,  or  Water  cement. 

39.  Common,  or  air  lime,  is  so  called  because  the  paste  made 
from  it  with  water  will  harden  only  in  the  air. 

40.  Hydraulic  lime  and  hydraulic  cement  both  take  their  name 
from  hardening  under  water.  The  former  differs  from  the  latter 
in  two  essential  points.  It  slakes  thoroughly,  like  common  lime, 
when  deprived  of  its  carbonic  acid,  and  it  does  not  harden 
promptly  under  water.  Hydraulic  cement,  on  the  contrary,  does 
not  slake,  and 'usually  hardens  very  soon. 

41.  Our  nomenclature,  with  regard  to  these  substances,  is  still 
quite  defective  for  scientific  arrangement.  For  the  lime-stones 
which  yield  hydraulic  lime  when  completely  calcined,  also  give 
an  hydraulic  cement  when  deprived  of  a  portion  only  of  their 
carbonic  acid  ;  and  other  lime-stones  yield,  on  calcination,  a  result 
which  can  neither  be  termed  lime  nor  hydraulic  cement,  owing 
to  its  slaking  very  imperfectly,  and  not  retaining  the  hardness 
which  it  quickly  takes  when  first  placed  under  water. 

M.  Vicat,  whose  able  researches  into  the  properties  of  lime  and 
mortars  are  so  well  known,  has  proposed  to  apply  the  term  cement 
lime-stones  (calcaires  a  ciment)  to  those  stones  which,  when  com- 
pletely calcined,  yield  hydraulic  cement,  and  which  under  no  de- 
gree of  calcination,  will  give  hydraulic  lime.  For  the  lime-stones 
which  yield  hydraulic  lime  when  completely  calcined,  and  which, 
when  subjected  to  a  degree  of  heat  insufficient  to  drive  off  all  their 
carbonic  acid,  yield  hydraulic  cement,  he  proposes  to  retain  the 
name  hydraulic  lime-stones ;  and  to  call  the  cement  obtained 
from  their  incomplete  calcination,  under-burnt  hydraulic  cement, 
(ciments  dHncuits,)  to  distinguish  it  from  that  obtained  from  the 
cement  stone.  With  respect  to  those  lime-stones  which,  by  cal- 
cination, give  a  result  that  partakes  partly  of  the  properties  both 
of  limes  and  cements,  he  proposes  for  them  the  name  of  dividing 
limes,  (chaux  limites.) 

The  terms  fat  and  meager  are  also  applied  to  limes  ;  owing  to 
the  difference  in  the  quality  of  the  paste  obtained  from  them  with 
the  same  quantity  of  water.  The  fat  limes  give  a  paste  which  is 
unctuous  both  to  the  sight  and  touch.  The  meager  limes  yield 
a  thin  paste.  These  names  were  of  some  importance  when  first 
introduced,  as  they  served  to  distinguish  common  from  hydraulic 
lime,  the  former  being  always  fat,  the  latter  meager ;  but,  later 
experience  having  shown  that  all  meager  limes  are  not  hydraulic, 
the  terms  are  no  longer  of  use,  except  to  designate  qualities  of 
the  paste  of  limes. 


LIME. 


15 


42.  Hydraulic  Limes  and  Cements.  The  lime-stonts  which 
yield  these  substances  are  either  argillaceous,  or  magnesian,  or 
argilo-magnesian.  The  products  of  their  calcination  vary  con- 
siderably in  their  hydraulic  properties.  Some  of  the  hydraulic 
limes  harden,  or  set  very  slowlv  under  water,  while  others  set  rap- 
idly. The  hydraulic  cements  set  in  a  very  short  time.  This 
diversity  in  the  hydraulic  energy  of  the  argillaceous  lime-stones 
arises  from  the  variable  proportions  in  which  the  lime  and  clav 
enter  into  their  composition. 

43.  M.  Petot,  a  civil  engineer  in  the  French  service,  in  an  able 
work  entitled  Recherches  sur  la  Chauffournerie,  gives  the  follow- 
ing table,  exhibiting  these  combinations,  and  the  results  obtained 
from  their  calcination. 


Lime. 

Clay. 

100 

0 

90 

10 

80 

20 

70 

30 

60 

40 

50 

50 

40 

60 

30 

70 

20 

80 

10 

90 

0 

100 

Resulting  products. 


Distinctive  characters  of  the  products. 


Very  fat  lime. 
Lime  a  little  hydraulic, 
do.    quite  hydraulic, 
do.  do. 

Plastic,  or  hydraulic  cement, 
do. 
do. 
Calcareous  puzzolano  (brick), 
do.  do. 

do.  do. 

Puzzolano  of  pure  clay  do. 


Incapable  of  hardening  in  water. 

C  Slakes  like   pure  lime,  when 

<     properly  calcined,  and  hard- 

f  ens  under  water. 
Does  not  slake  under  any  cir- 
cumstances, and  hardens  un- 
der water  with  rapidity. 
Does  not  slake  nor  harden  un- 
der water,  unless  mixed  with 
a  fat,  or  an  hydraulic  lime. 

Same  as  the  preceding. 


44.  The  most  celebrated  European  hydraulic  cements  are  ob- 
tained from  argillaceous  lime-stones,  which  vary  but  slightly  in 
their  constituent  elements  and  properties.  The  following  table 
gives  the  results  of  analyses  to  determine  the  relative  proportions 
of  lime  and  clay  in  these  cements. 


Table  of  Foreign  Hydraulic  Cements,  showing  the  relative  pro- 
portions of  Clay  and  Lime  contained  in  them. 


LOCALITY. 

Lime. 

Clay. 

English,  {commonly  known  as  Parker's,  or  Roman  cement) 
French,  (made  from  Boulogne  pebbles) 

Dc.     (Pouilly) 

Do.          do.                

Do.     (Baye)      ........ 

i 

55.40 
54.00 
42.86 
36.37 
21  62 
62.00 

44.60 
46.00 
57.14 
63.63 
78.38 
38.00 

The  hydraulic  cements  used  in  England  are  obtained  frofl* 


16 


BUILDING  MATERIALS. 


various  localities,  and  differ  but  little  in  the  relative  proportion* 
of  lime  and  clay  found  in  them.  Parker's  cement,  so  called  from 
the  name  of  the  person  who  first  introduced  it,  is  obtained  by 
calcining  nodules  of  septaria.  The  composition  of  these  nodules 
is  the  same  as  that  of  the  Boulogne  pebbles  found  on  the  opposite 
coast  of  France.  The  stones  which  furnish  the  English  and 
French  hydraulic  cements,  contain  but  a  very  small  amount  of 
magnesia. 

45.  The  best  known  hydraulic  cements  of  the  United  States, 
are  manufactured  in  the  State  of  New  York.  The  following 
analyses  of  some  of  the  hydraulic  lime-stones,  from  the  most 
noted  localities,  published  in  the  Geological  Report  of  the  State 
of  New  York,  1839,  are  given  by  Dr.  Beck. 


Analysis  of  the  Manlius  Hydraulic  Lime-stone. 

Carbonic  acid 39.80 

Lime 26.24 

Magnesia 18.80 

Silica  and  alumina      ....  13.50 

Oxide  of  iron 1.25 

Moisture  and  loss        ....  1.41 


100.00 


This  stone  belongs  to  the  same  bed  which  yields  the  hydraulic 
cement  obtained  near  Kingston,  in  Upper  Canada. 


Analysis  of  the  Chittenango  Hydraulic  Lime-stone,  before  and 
after  calcination. 


Unburn  t. 

Carbonic  acid  and  moisture 

Lime 

Magnesia      .... 

Silica 

Alumina  and  oxide  of  iron 

Burnt. 

Carbonic  acid 

Lime     .... 

Magnesia     . 
Silica    . 
Aluirina 
Peroxide  of  iror 
Moisture 

39.33 

25.00 

17.83 

11.76 

2.73 

1.50 

1.50 

10.90 
39.50 
22.27 
16.56 
10.77 

100.00 

100.00 

LIME. 


17 


Analysis  of  the  Hydraulic  Lime-stc  .e  from  Ulster  Co.,  along 
the  line  of  the  Delaware  and  Hudson  Canal,  before  and  after 
burning. 


Un  burnt. 

Burnt 

34.20 

5 

25.50 

37.60 

Magnesia 

12.35 

16. 65 

15.37 

22.75 

Alumina 

9.13 

13.40 

Oxide  of  iron    ..... 

2.25 

3.30 

Bituminous  matter,  moisture,  and  loss 

1.20 

1.30 

100.00 

100.00 

The  hydraulic  cement  from  this  last  locality  has  become  gen- 
erally well  known,  having  been  successfully  used  for  most  of  the 
military  and  civil  public  works  on  the  sea-board. 

From  the  results  of  the  analyses  of  all  the  above  limestones,  it 
appears  that  the  proportions  of  lime  and  clay  contained  in  them 
place  them  under  the  head  of  hydraulic  cements,  according  to  the 
classification  of  M.  Petot.  They  do  not  slake,  and  they  all  set 
rapidly  under  water. 

46.  The  discovery  of  the  hydraulic  properties  of  certain  mag 
nesian  lime-stones  is  of  recent  date,  and  is  due  to  M.  Vicat,  who 
first  drew  attention  to  the  subject.     M.  Vicat  inclines  to  the 
opinion,  that  magnesia  alone,  without  the  presence  of  some  clay, 
will  yield  only  a  feeble  hydraulic  lime.     He  states,  that  he  has 
never  been  able  to  obtain  any  other,  from  proceeding  synthetically 
with  common  lime  and  magnesia ;  and  that  he  knows  of  no  well- 
authenticated  instance  in  which  any  of  the  dolomites,  either  of 
the  primitive  or  transition  formations,  have  yielded  a  good  hydrau 
lie  lime.    The  stones  from  these  formations,  he  states,  are  devoid 
of  clay ;  being  very  pure  crystalline  carbonates,  or  else  contain 
silex  only  in  the  state  of  fine  sand.     From  M.  Vicat's  experi- 
ments, it  is  rendered  certain  that  carbonate  of  marnesia  in  combi- 
nation with  carbonate  of  lime,  in  the  proportion  of  4 <J  parts  of  the 
latter  to  from  30  to  40  of  the  former,  will  produce  a  feebly  hy 
draulic  lime,  which  does  not  appear  to  increase  in  hardness  after 
it  has  once  set ;  but  that  with  the  same  proportions,  some  hup 
dredths  of  clay  are  requisite  to  give  hydraulic  energy  to  the  com- 
pound.    This  proportion  of  clay  M.  Vicat  supposes  may  cause 
the  formation  of  triple  hydro-silicates  of  lime,  alumina,  and  mag 
nesia,  having  all  the  characteristic  properties  of  good  hydraulic 
lime. 

47.  The  hydraulic  properties  of  the  magnesian  lime-stones  of 


18 


BUILDING  MATERIALS. 


the  United  States  were  noticed  by  Professor  W.  B.  Rogers,  wht , 
in  his  Report  of  the  Geological  Survey  of  Virginia,  1 838.  has 
gjven  the  following  analyses  of  some  of  the  stones  from  different 
localities. 


Carbonate  of  lime    .... 
Carbonate  of  magnesia     . 
Alumina  and  oxide  of  iron 
Silica  and  insoluble  matter 

Loss 

No.  1. 

No.  2. 

No.  3. 

No  4. 

65  60 
3U.20 
1.50 
2.50 
0.40 
0.60 

53.23 
41.00 
0.80 
2.80 
0.40 
1.77 

48.20 

35.76 

1.20 

12.10 

2.73 

0.01 

55.03 

24.16 

2.60 

15.30 

1.20 

1.71 

100.00 

100.00 

100.00 

100.00 

The  lime-stone  No.  1  of  the  above  table  is  from  Sheppardstown 
on  the  Potomac,  in  Virginia ;  it  is  extensively  manufactured  for 
hydraulic  cement.  No.  2  is  from  the  Natural  Bridge,  and  banks 
of  Cedar  Creek,  Virginia ;  it  makes  a  good  hydraulic  cement. 
No.  3  is  from  New  York,  and  is  extensively  burnt  for  cement. 
No.  4  is  from  Louisville,  Kentucky;  said  to  make  a  good  cement. 

48.  M.  Vicat  states,  that  a  magnesian  lime-stone  of  France 
containing  the  following  constituents,  lime  40  parts,  magnesia  21, 
and  silica  21,  yields  a  good  hydraulic  cement ;  and  he  gives  the 
following  analysis  of  a  stone  which  gives  a  good  hydraulic  lime. 

Carbonate  of  lime       ....  50.60 

Carbonate  of  magnesia        .         .         .  42.00 

Silica 5.00 

Alumina 2.00 

Oxide  of  iron 0.40 


100.00 


By  comparing  the  constituents  of  these  two  last  stones  with  the 
analyses  of  the  cement-stones  of  New  York,  and  the  magnesian 
hydraulic  lime-stones  of  Prof.  Rogers,  it  will  be  seen  that  they 
consist,  respectively,  of  nearly  the  same  combinations  of  lime, 
magnesia,  and  silica. 

49.  Physical  Characters  and  Tests  of  Hydraulic  Lime-stones. 
The  simple  external  characters  of  a  lime-stone,  as  color,  texture, 
fracture,  and  taste,  are  insufficient  to  enable  a  person  to  decide 
whether  it  belongs  to  the  hydraulic  class ;  although  they  assist 
conjecture,  particularly  if  the  rock,  from  which  the  specimen  is 
taken,  is  found  in  connection  with  the  clay  deposites,  or  if  it  be- 
long to  a  stratum  wb  >se  general  level  and  characteristics  are  the 


LIME.  19 

same  as  the  argilo-magnesian  rocks.  These  rocks  are  generally 
some  shade  of  drab,  or  of  gray,  or  of  a  dark  grayish  blue  ;  have 
a  compact  texture  ;  fracture  even  or  conchoidal ;  with  a  clayey  or 
earthy  smell  and  taste.  Although  the  hydraulic  lime-stones  are 
usually  colored,  still  it  may  happen  that  tb  t  stone  may  be  of  a  pure 
white,  arising  from  the  combination  of  lime  with  a  pure  clay. 

The  difficulty  of  pronouncing  upon  the  class  to  which  a  lime- 
stone belongs,  from  its  physical  properties  alone,  renders  it  neces- 
sary to  resort  to  a  chemical  analysis,  and  even  to  direct  experiment, 
to  decide  the  question. 

50.  In  making  a  complete  chemical  analysis  of  a  lime-stone,  more 
skill  in  chemical  manipulations  is  requisite  than  engineers  usually 
possess ;  but  a  person  who  has  the  ordinary  elementary  know- 
ledge of  chemistry,  can  readily  ascertain  the  quantity  of  clay  or 
of  magnesia  contained  in  a  lime-stone,  and  from  these  two  ele- 
ments can  pronounce,  with  tolerable  certainty,  upon  its  hydraulic 
properties.  To  arrive  at  this  conclusion,  a  small  portion  of  the 
stone  to  be  tested — about  five  drachms — is  taken  and  reduced  to 
a  powder ;  this  is  placed  in  a  capsule,  or  an  ordinary  watch 
crystal,  and  slightly  diluted  muriatic  acid  is  poured  over  it  until 
it  ceases  to  effervesce.  The  capsule  is  then  gently  heated,  and 
the  liquor  evaporated,  until  the  residue  in  the  capsule  has  acquired 
the  consistence  of  thin  paste.  This  paste  is  thrown  into  a  pint 
of  pure  water  and  well  shaken  up,  and  the  mixture  is  then  fil- 
tered. The  residue  left  on  the  filtering  paper  is  thoroughly  dried, 
by  bringing  it  to  a  red  heat ;  this  being  weighed  will  give  the 
clay,  or  insoluble  matter,  contained  in  the  stone.  It  is  important 
to  ascertain  the  state  of  mechanical  division  of  the  insoluble  mat 
ter  thus  obtained  ;  for  if  it  be  wholly  granular,  the  stone  will  not 
yield  hydraulic  lime.  The  granular  portion  must  therefore  be 
carefully  separated  from  the  other  before  the  latter  is  dried  and 
weighed. 

51.  If  the  sample  tested  contains  magnesia,  an  indication  of 
this  will  be  given  by  the  slowness  with  which  the  acid  acts  ;  if 
the  quantity  of  magnesia  be  but  little,  the  solution  will  at  first 
proceed  rapidly  and  then  become  more  sluggish.  To  ascertain 
the  quantity  of  magnesia,  clear  lime-water  must  be  added  to  the 
filtered  solution  as  long  as  any  precipitate  is  formed,  and  this 
precipitate  must  be  quickly  gathered  on  filtering  paper,  and  then 
be  washed  with  pure  water.  The  residue  from  this  washing  is 
the  magnesia.  It  must  be  thoroughly  dried  before  being  weighed, 
to  ascertain  its  proportion  to  the  clay. 

52.  Having  ascertained,  by  the  preceding  analysis,  the  proba- 
ble hydraulic  energy  of  the  stone,  a  sample  of  it  should  also  be 
submitted  to  direct  experiment.  This  may  be  likewise  done  or, 
t  small  scale.     A  sample  of  the  stone  n<  ist  be  reduced  to  fras; 


20  BUILDING  MATERIALS 

ments  about  the  size  of  a  walnut.  A  crucible,  perforated  with 
holes  for  the  free  admission  of  air,  is  filled  with  these  fragments, 
and  placed  over  a  fire  sufficiently  powerful  to  drive  off  the  car- 
bonic acid  of  the  stone.  The  time  for  effecting  this  will  depend 
on  the  intensity  of  the  heat.  When  the  heat  has  been  applied 
for  three  or  four  hours,  a  small  portion  of  the  calcined  stone  may 
be  tried  with  an  acid,  and  the  degree  of  the  calcination  may  be 
judged  of  by  the  more  or  less  copiousness  of  the  effervescence 
that  ensues.  If  no  effervescence  takes  place,  the  operation  may 
be  considered  completed.  The  calcined  stone  should  be  tiied 
soon  after  it  has  become  cold  ;  otherwise,  it  should  be  kept  in 
a  glass  jar  made  as  air-tight  as  practicable  until  used. 

53.  When  the  calcined  stone  is  to  be  tried,  it  is  first  slaked 
by  placing  it  in  a  small  basket,  which  is  immersed  for  five  or  six 
seconds  in  pure  water.  The  stone  is  emptied  from  the  basket  so 
soon  as  the  water  has  drained  off,  and  is  allowed  to  stand  until 
the  slaking  is  terminated.  This  process  will  proceed  more  or 
less  rapidly,  according  to  the  quality  of  the  stone,  and  the  degree 
of  its  calcination.  In  some  cases,  it  will  be  completed  in  a  few 
minutes  ;  in  others,  portions  only  of  the  stone  will  fall  to  powder, 
the  rest  crumbling  into  lumps  which  slake  very  sluggishly  ;  while 
other-varieties,  as  the  true  cement  stones,  give  no  evidence  of  slak- 
ing. If  the  stone  slakes  either  completely  or  partially,  it  must  be 
converted  into  a  paste  of  the  consistence  of  soft  putty,  being  ground 
up  thoroughly,  if  necessary,  in  an  iron  mortar.  The  paste  is 
made  into  a  cake,  and  placed  on  the  bottom  of  an  ordinary  tum- 
bler, care  being  taken  to  make  the  diameter  of  the  cake  the  same 
as  that  of  the  tumbler,  which  is  filled  with  water,  and  the  time  of 
Immersion  noted.  It  the  lime  is  only  moderately  hydraulic,  it 
will  have  become  hard  enough  at  the  end  of  fifteen  or  twenty 
days,  to  resist  the  pressure  of  the  finger,  and  will  continue  to 
harden  slowly,  more  particularly  from  the  sixth  or  eighth  month 
after  immersion  ;  and  at  the  end  of  a  year  it  will  have  acquired 
the  consistency  of  hard  soap,  and  will  dissolve  slowly  in  pure 
water.  A  fair  hydraulic  lime  will  have  hardened  so  as  to  resist 
the  pressure  of  the  finger,  in  about  six  or  eight  days  after  immer- 
sion, and  will  continue  to  grow  harder  until  from  six  to  twelve 
months  after  immersion  ;  it  will  then  have  acquired  the  hardness 
of  the  softest  calcareous  stones,  and  will  be  no  longer  soluble  in 
pure  water.  When  the  stone  is  eminently  hydraulic,  it  will  have 
become  hard  in  from  two  to  four  days  after  immersion,  and  in  one 
month  it  will  be  quite  hard  and  insoluble  in  pure  water ;  after  six 
months,  its  hardness  will  be  abou*  equal  to  the  more  absorbent 
calcareous  stones ;  will  splinter  from  a  blow,  presenting  a  slaty 
fracture. 

As  the  hyd  raulic  cements  Jo  not  slake  perceptibly,  the  burnt 


LIME.  21 

stone  must  first  be  reduced  to  a  fine  powder  before  it  is  made 
mto  a  paste.  The  paste,  when  kneaded  between  the  fingers,  be- 
comes warm,  and  will  generally  set  in  a  few  minutes,  either  in  the 
open  air  or  in  water.  Hydraulic  cement  is  far  more  sparingly 
soluble  in  pure  water  than  the  hydraulic  lime ;  and  the  action  of 
pure  water  upon  them  ceases,  apparently,  after  a  few  weeks  im- 
mersion in  it. 

54.  Calcination  of  Lime-stone.  The  effect  of  heat  on  lime- 
stones varies  with  the  constituent  elements  of  the  stone.  The 
pure  lime-stones  will  stand  a  high  degree  of  temperature  with- 
out fusing,  losing  only  their  carbonic  acid  and  water.  The  im- 
pure stones  containing  silica  fuse  completely  under  a  great  heat, 
and  become  more  or  less  vitrified  when  the  temperature  much  ex- 
ceeds a  red  heat.  The  action  of  heat  on  the  impure  lime-stones, 
besides  driving  off  their  carbonic  acid  and  water,  modifies  the  re- 
lations of  their  other  chemical  constituents.  The  argillaceous 
stones,  for  example,  yield  an  insoluble  precipitate  when  acted  on 
by  an  acid  before  calcination,  but  are  perfectly  soluble  afterwards, 
unless  the  silex  they  contain  happens  to  be  in  the  form  of  grains. 

55.  The  calcination  of  the  hydraulic  lime-stones,  from  their 
fusible  nature,  requires  to  be  conducted  with  great  care ;  for,  if 
not  pushed  far  enough,  the  under-burnt  portions  will  not  Slake  ; 
and,  if  carried  too  far,  the  stone  becomes  dead  or  sluggish ;  slakes 
very  slowly  and  imperfectly  at  first ;  and,  if  used  in  this  state  for 
masonry,  may  do  injury  by  the  swelling  which  accompanies  the 
after-slaking. 

56.  The  more  or  less  facility  with  which  the  impure  lime-stones 
can  be  burned,  depends  upon  several  causes  ;  as  the  compactness 
of  the  stone  ;  the  size  of  the  fragments  submitted  to  heat ;  and 
the  presence  of  a  current  of  air,  or  of  aqueous  vapor.  The  more 
compact  stones  yield  their  carbonic  acid  less  readily  than  those 
of  an  opposite  texture.  Stones  which,  when  broken  into  very 
small  lumps,  can  be  calcined  under  the  red  heat  of  an  ordinary 
fire  in  a  few  hours,  will  require  a  far  greater  degree  of  tempera- 
ture, and  for  a  much  longer  period,  when  broken  into  fragments 
of  six  or  eight  inches  in  diameter.  This  is  particularly  the  case 
with  the  impure  lime-stones,  which,  when  in  large  lumps,  vitrify 
at  the  surface  before  the  interior  is  thoroughly  burnt. 

57.  If  a  current  of  vapor  is  passed  over  the  stone  aftei  it  has 
commenced  to  give  off  its  carbonic  acid,  the  remaining  portion  of 
the  gas  which,  under  ordinary  circumstances,  is  expelled  with 
great  difficulty,  particularly  near  the  end  of  the  process  of  calci- 
nation, will  be  carried  off  much  sooner.  This  influence  of  an 
aqueous  current  is  attributed,  by  M.  Gay-Lussac,  purely  to  a 
mechanical  action,  by  removing  the  gas  as  it  is  evolved,  and  his 
experim'  us  go  to  show  that  a  like  effect  is  produced  by  an  at 


22 


BUILDING  MATERIALS. 


mospheric  current.  In  burning  the  impure  lime-stones,  however 
an  aqueous  current  produces  the  farther  beneficial  effect  of  pre 
venting  the  vitrification  of  the  stone,  when  the  temperature  has 
become  too  elevated ;  but  as  the  vapor,  on  coming  in  contact 
with  the  heated  stone,  carries  off  a  large  portion  of  the  heat,  this, 
together  with  the  latent  heat  contained  in  it,  may  render  its  use 
in  some  cases,  far  from  economical. 

58.  Wood,  charcoal,  peat,  the  bituminous  and  anthracite  coals 
are  used  for  fuel  in  lime-burning.  M.  Vicat  states,  that  wood  is 
the  best  fuel  for  burning  hydraulic  lime-stones  ;  that  charcoal  is 
inferior  to  bituminous  coal ;  and  that  the  results  from  this  last  are 
very  uncertain.  When  wood  is  used,  it  should  be  dry  and  split 
up,  to  burn  quickly  and  give  a  clear  blaze.  The  common  opinion 
among  lime-burners,  that,  the  greener  the  fuel  the  better,  and  that 
the  lime-stone  should  be  watered  before  it  is  placed  in  the  kiln,  is 
wrong  ;  as  a  large  portion  of  the  heat  is  consumed  in  converting 
the  water  in  both  cases  into  vapor.  Coal  is  a  more  economical 
fuel  than  wood,  and  is  therefore  generally  preferred  to  it ;  but  it 
requires  particular  care  in  ascertaining  the  proper  quantity  for  use. 

59.  Lime-kilns.  Great  diversity  is  met  with  in  the  forms  and 
proportions  of  lime-kilns.  Wherever  attention  has  been  paid  to 
economy  in  fuel,  the  cylindrical,  ovoidal,  or  the  inverted  conical 
form  has  been  adopted.  The  two  first  being  preferred  for  wood, 
and  the  last  for  coal. 

60.  The  whole  of  the  burnt  lime  is  either  drawn  from  the  kiln 
at  once,  or  else  the  burning  is  so  regulated,  that  fresh  stone  and 
fuel  are  added  as  the  calcined  portions  are  withdrawn.  The  lat- 
ter method  is  usually  followed  when  the  fuel  used  is  coal.  The 
stone  and  coal,  broken  into  proper  sizes,  (Fig.  1,)  and  in  propor- 


Fig.  1  represents  a  vertical  section  through  the  axis  and  centre  lines 

of  the  entrances  communicating  with  the  interior  of  a  kiln  for 

burning  lime  with  coal. 
A,  solid  masonry  of  the  kiln,  which  is  built  up  on  the  exterior  like  a 

square  tower,  with  two  arched  entrances  at  B,  B  on  opposite 

sides. 

C,  interior  of  the  kiln,  lined  with  fire-brick  or  stone. 

D,  ash-pit. 

c,  c,  openings  between  B,  B  and  the  interior  through  which  the  burnt 
lime  is  drawn. 


tions  determined  by  experiment,  are  placed  in  the  kiln  in  alternate 
layers ;  the  coal  is  ignited  at  the  bottom  of  the  kiln,  and  fresh 
strata  are  added  at  the  top,  as  the  burnt  mass  settles  down  and  is 
partially  withdrawn  at  the  bottom.  Kilns  used  in  this  way  are 
called  perpetual  kilns ;  they  are  more  economical  in  toe  con 
sumption  of  fuel  than  those  in  which  the  burning  is  intermitted 
an  J  which  are,  on  this  account,  termed  intermittent  -itlv*.    Wor 


w 

V'v 

A;  / 

\X 

\ 

J 

;  1 

c 

.  1 

■f  i 

r    i 

1 

wb-^ 

1  \ 

wk^ 

»m 

\\ 

*  v| 

it 

(d) 

LIME. 


23 


mav  also  be  used  as  fuel  in  perpetual  kiln-,  but  not  with  such 
economy  as  coal ;  it  moreover  presents  many  inconveniences,  in 
supplying  the  kiln  with  fresh  stone,  and  in  regulating  its  dis- 
charge. The  inverted  conical-shaped  kiln  is  generally  adopted 
for  coal,  and  the  ovoidal-shaped  for  wood. 

61.  Some  care  is  requisite  in  filling  the  kiln  with  stone  when  a 
wood  fire  is  used.   A  dome  (Fig.  2)  is  formed  of  the  largest  blocks 


Fig.  2  represents  a  vertical  section  through  the 
axis  and  centre  line  of  the  entrance  of  a  lime- 
kiln for  wood. 

A,  solid  masonry  of  the  kiln. 

Bt  arched  entrance. 

C,  doorway  for  drawing  kiln  and  supplying  fuel. 

D,  interior  of  kiln. 

E,  dome  of  broken  stone,  shown  by  the  dotted 
line. 


>\\\<S5SiKKK!SK^^^ 


of  the  broken  stone,  which  either  rests  on  the  bottom  of  the  kiln  or 
on  the  ash-grate.  The  lower  diameter  of  the  dome  is  a  few  feet 
less  than  that  of  the  kiln  ;  and  its  interior  is  made  sufficiently  capa- 
cious to  receive  the  fuel  which,  cut  into  short  lengths,  is  placed 
up  endwise  around  the  dome.  The  stone  is  placed  over  and 
around  the  courses  which  form  the  dome,  the  largest  blocks  in 
the  centre  of  the  kiln.  The  management  of  the  fire  is  a  matter 
of  experiment.  For  the  first  eight  or  ten  hours  it  should  be  care- 
fully regulated,  in  order  to  bring  the  stone  gradually  to  a  red  heat. 
By  applying  a  high  heat  at  first,  or  by  any  sudden  increase  of  it 
until  the  mass  has  reached  a  nearly  uniform  temperature,  the 
stone  is  apt  to  shiver,  and  choke  the  kiln,  by  stopping  the  voids 
between  the  courses  of  stone  which  form  the  dome.  After  the 
stone  is  brought  to  a  red  heat,  the  supply  of  fuel  should  be  uni- 
form until  the  end  of  the  calcination.  The  practice  sometimes 
adopted,  of  abating  the  fire  towards  the  end,  is  bad,  as  the  last 
portions  of  carbonic  acid  retained  by  the  stone,  require  a  high  de- 
gree of  heat  for  their  expulsion.  The  indications  of  complete 
calcination  are  generally  manifested  by  the  diminution  which 
gradually  takes  place  in  the  mass,  and  which,  at  this  stage,  is 
about  one  sixth  of  the  primitive  volume ;  by  the  broken  appear- 
ance of  the  stone  which  forms  the  dome,  the  interstices  between 
which  being  also  choked  up  by  fragments  of  the  burnt  stone  ;  and 
by  the  ease  with  which  an  iron  bar  may  be  forced  down  through 
the  burnt  stone  in  the  kiln.  When  these  indications  of  complete 
calcination  are  observed,  the  kiln  should  be  closed  for  ten  or 
twelve  hours,  to  confine  the  heat  and  finish  the  burning  of  the  up- 
per strata. 

62.  The  form  and  relative  dimensions  of  a  kiln  for  wood  can 


IA  BUILDING  MATERIALS. 

De  determined  only  by  careful  experiment.  If  too  great  height 
be  given  to  the  mass,  the  lower  portions  may  be  overburncd  be- 
fore the  upper  are  burned  enough.  The  proportions  between  the 
height  and  mean  horizontal  section,  will  depend  on  the  texture  of 
the  stone ;  the  size  of  the  fragments  into  which  it  is  broken  for 
burning ;  and  the  more  or  less  facility  with  which  it  vitrifies.  In 
the  memoir  of  M.  Petot,  already  cited,  it  is  stated  as  the  results 
of  experiments  made  at  Brest,  tint  large-sized  kilns  are  more 
economical,  both  in  the  consumption  of  fuel  and  in  the  cost  of 
attendance,  than  small  ones  ;  but  that  there  is  no  notable  econo- 
my in  fuel  when  the  mean  horizontal  section  of  the  kiln  exceeds 
sixty  square  feet. 

63.  The  circular  seems  the  most  suitable  form  for  the  horizon- 
tal sections  of  a  kiln,  both  for  strength  and  for  economizing  the 
heat.  Were  the  section  the  same  throughout,  or  the  form  of  the 
interior  of  the  kiln  cylindrical,  the  strata  of  stone,  above  a  certain 
point,  would  be  very  imperfectly  burned  when  the  lower  were 
enough  so,  owing  to  the  rapidity  with  which  the  inflamed  gases, 
arising  from  the  combustion,  are  cooled  by  coming  into  contact 
with  the  stone.  To  procure,  therefore,  a  temperature  throughout 
the  heated  mass  which  shall  be  nearly  uniform,  the  horizontal  sec- 
tions of  the  kiln  should  gradually  decrease  from  the  point  where 
the  flame  rises,  which  is  near  the  top  of  the  dome  of  broken  stone, 
to  the  top  of  the  kiln.  This  contraction  of  the  horizontal  section, 
from  the  bottom  upward,  should  not  be  made  too  rapidly,  as  the 
draft  would  be  injured,  and  the  capacity  of  the  kiln  too  much 
diminished  ;  and  in  no  case  should  the  area  of  the  top  opening  be 
less  than  about  one  fourth  the  area  of  the  section  taken  near  the 
top  of  the  dome.  The  best  manner  of  arranging  the  sides  of  the 
kiln,  in  the  plane  of  the  longitudinal  section,  is  to  connect  the  top 
opening  with  the  horizontal  section  through  the  top  of  the  dome, 
by  an  arc  of  a  circle  whose  tangent  at  the  lower  point  shall  be 
vertical. 

64.  Lime-kilns  are  constructed  either  of  brick,  or  of  some  of 
the  more  refractory  stones.  The  walls  of  the  kiln  should  be  suf- 
ficiently thick  to  confine  the  heat,  and,  when  the  locality  admits 
of  it,  they  are  built  into  a  side  hill ;  otherwise,  it  may  be  neces- 
sary to  use  iron  hoops,  and  vertical  bars  of  iron,  to  strengthen  the 
brick-work.  The  interior  of  the  kiln  should  be  faced  either  with 
good  fire-brick  or  with  fire-stone. 

65.  M.  Petot  prefers  kilns  arranged  with  fire-grates,  and  an 
ash-pit.  under  the  dome  of  broken  stone,  for  the  reason  that  they 
give  the  means  of  better  regulating  the  heat,  and  of  throwing  the 
flame  more  in  the  axis  of  the  kiln  than  can  be  done  in  kilns  with 
out  them.  The  action  of  the  flame  is  thus  more  uniformly  felt 
through  the  mass  o?  st<  ne  above  the  top  of  the  dome,  while  that 


LIME. 


25 


of  the  radiated  heat  upon  the  stone  around  the  dome,  is  also  more 
uniform . 

G6.  M.  Petot  states,  that  the  height  of  the  mass  of  stone  above 
the  top  of  the  dome  should  not  be  greater  than  from  ten  to  thir- 
teen feet,  depending  on  the  more  or  less  compact  texture  of  the 
stone,  and  the  more  or  less  ease  with  which  it  vitrifies.  He  pro- 
poses to  use  kilns  with  two  stories,  (Fig.  3,)  for  the  purpose 


Fig.  3  represents  a  vertical  section 
through  the  axis  and  centre  line  of 
the  entrance  of  a  lime-kiln  with  two 
stories  for  wood. 

A,  solid  masonry  of  the  kiln. 

B,  dome  shown  by  the  dotted  line. 

C,  interior  of  lower  story. 

D,  dome  of  upper  story. 

E,  interior  of  upper  story. 

a,  arched  entrance  to  kiln. 

b,  receptacle  for  water  to  furnish  a 
current  of  aqueous  vapor. 

c,  doorway  for  drawing  kiln,  &c, 
closed  by  a  fire-proof  door. 

J,  ash-pit  under  fire-grate. 

e,  upper  doorway  for  drawing  kiln,  &c 


of  economizing  the  fuel,  by  using  the  heat  which  passes  off  from 
the  top  of  the  lower  story,  and  would  otherwise  be  lost,  to  heat 
the  stone  in  the  upper  story ;  this  story  being  arranged  with  a 
side-door,  to  introduce  fuel  under  its  dome  of  broken  stone,  and 
complete  the  calcination  when  that  of  the  stone  in  the  lower 
story  is  finished. 

M.  Petot  gives  the  following  general  directions  for  regulating 
the  relative  dimensions  of  the  parts  of  the  kiln.  The  greatest 
horizontal  section  of  the  kiln  is  placed  rather  below  the  top  of  the 
dome  of  broken  stone  ;  the  diameter  of  this  section  being  1.82,  the 
diameter  of  the  grate.  The  height  of  the  dome  above  the  grate 
is  from  3  to  6  feet,  according  to  the  quantity  of  fuel  to  be  con- 
sumed hourly.  The  bottom  of  the  kiln,  on  which  the  piers  of  the 
dome  rest,  is  from  4  to  6  inches  above  the  top  of  the  grate  ;  the 
diameter  of  the  kiln  at  this  point  being  about  2  feet  9  inches 
greater  than  that  of  the  grate.  The  diameter  of  the  horizontal 
section  at  top  is  0.63,  the  diameter  of  the  greatest  horizontal  sec- 
tion. The  horizontal  sections  of  the  kiln  diminish  from  the  section 
near  the  top  of  the  dome  to  the  top  and  bottom  of  the  kiln  ;  the 
sides  of  the  kiln  receiving  the  form  shown  in  Fig.  3  :  the  object 
of  contracting  the  kiln  towards  the  bottom  being  to  allow  the  stone 
near  the  bottom  of  the  kiln  to  be  thoroughly  burned  by  the  radiated 
heat      The  grate  is  formed  of  cast-iron  bars  of  the  usual  form 


86  BUILDINC'   alATBRIALS. 

tlie  area  of  the  spaces  between  the  bars  being  one  fourth  the  total 
area  of  the  grate.  The  bottom  of  the  ash-pit,  which  may  be  on 
the  same  level  as  the  exterior  ground,  is  placed  18  inches  below 
the  grate ;  and  at  the  entrance  to  the  ash-pit  is  placed  a  reservoir 
for  water,  about  1 8  inches  in  depth,  to  furnish  an  aqueous  cur- 
rent. The  draft  through  the  grate  is  regulated  by  a  lateral  air- 
channel  to  the  ash-pit,  which  can  be  totally  or  partially  shut  by  a 
valve ;  the  area  of  the  cross  section  of  this  channel  is  one  tenth 
the  total  area  of  the  grate.  A  square  opening  16  inches  wide, 
the  bottom  of  which  is  on  a  level  with  the  bottom  of  the  kiln, 
leads  to  the  dome  for  the  supply  of  the  fuel.  This  opening  is 
closed  with  a  fire-proof  and  air-tight  door. 

In  arranging  a  kiln  with  two  stories,  M.  Petot  states,  that  the 
grates  of  the  upper  story  are  so  soon  destroyed  by  the  heat,  that 
it  is  better  to  suppress  them,  and  to  place  the  fuel  for  completing 
the  calcination  of  the  stone  of  this  story,  on  the  top  of  the  burnt 
stone  of  the  lower  story. 

67.  Slaking  Lime.  Quick-lime  may  be  slaked  in  three  dif- 
ferent ways.  By  pouring  sufficient  water  on  the  burnt  stone  to 
convert  the  slaked  lime  into  a  thin  paste,  which  is  termed  drown- 
ing the  lime.  By  placing  the  burnt  stone  in  a  basket,  and  im 
mersing  it  for  a  few  seconds  in  water,  during  which  ti  me  it  will 
imbibe  enough  water  to  cause  it  to  fall,  by  slaking,  into  a  dry 
powder  ;  or  by  sprinkling  the  burnt  stone  with  a  sufficient  quan 
tity  of  water  to  produce  the  same  effect.  By  allowing  the  stone 
to  slake  spontaneously,  from  the  moisture  it  imbibes  from  the 
atmosphere,  which  is  termed  air-slaking. 

68.  Opinion  seems  to  be  settled  among  engineers,  that  drown- 
ing is  the  worst  method  of  slaking  lime  which  is  to  be  used  for 
mortars.  When  properly  done,  however,  it  produces  a  finer  paste 
than  either  of  the  other  methods  ;  and  it  may  therefore  be  resorted 
to  whenever  a  paste  of  this  character,  or  a  whitewash  is  wanted. 
Some  care,  however,  is  requisite  to  produce  this  result.  The 
stone  should  be  fresh  from  the  kiln,  otherwise  it  is  apt  to  slake 
into  lumps  or  fine  grit.  All  the  water  used  should  be  poured 
over  the  stone  at  once,  which  should  be  arranged  in  a  basin  or 
vessel,  so  that  the  water  surrounding  it  may  be  gradually  imbibed 
as  the  slaking  proceeds.  If  fresh  water  be  added  during  the  slak- 
ing, it  checks  the  process,  and  causes  a  gritty  paste  to  form. 

69.  In  slaking  by  immersion,  or  by  sprinkling  with  water,  the 
stone  should  be  reduced  to  small-sized  fragments,  otherwise  the 
slaking  will  not  proceed  uniformly.  The  fat  limes  should  be  in 
lumps,  about  the  size  of  a  walnut,  for  immersion ;  and,  when 
withdrawn  from  the  water,  should  be  placed  immediately  in  bins 
or  be  covered  with  sand,  to  confine  the  heat  and  vapor.  If  left 
exposed  to  the  air,  the  lime  becomes  chilled  and  separates  into  a 


LIME.  27 

coarse  grit,  which  takes  some  time  to  slake  thoroughly  when 
more  water  is  added.  Sprinkling  the  lime  is  a  more  convenient 
process  than  immersion,  and  is  equally  good.  To  effect  the  slak 
ing  in  this  way,  the  stone  should  be  broken  into  fragments  of  a 
suitable  size,  which  experiment  will  determine,  and  be  placed  in 
small  heaps,  surrounded  by  sufficient  sand  to  cover  them  up  when 
the  slaking  is  nearly  completed.  The  stone  is  then  sprinkled 
with  about  one  fourth  its  bulk  of  water,  poured  through  the  rose 
of  a  watering-pot,  those  lumps  which  seem  to  slake  most  slug- 
gishly receiving  the  most  water ;  when  the  process  seems  com- 
pleted, the  heap  is  carefully  covered  over  with  the  sand,  and 
allowed  to  remain  a  day  or  two  before  it  is  used. 

70.  Slaking  either  by  immersion  or  by  sprinkling  is  considered 
the  best.  The  quantity  of  water  imbibed  by  lime  when  slaked 
by  immersion,  varies  with  the  nature  of  the  lime  ;  100  parts  of 
fat  lime  will  take  up  only  1 8  parts  of  water  ;  and  the  same  quan- 
tity of  meager  lime  will  imbibe  from  20  to  35  parts.  One  volume, 
in  powder,  of  the  burnt  stone  of  rich  lime  yields  from  1.50  to 
1 .70  in  volume  of  powder  of  slaked  lime ;  while  one  volume 
of  meager  lime,  under  like  circumstances,  will  yield  from  1 .80  to 
2.1^  in  volume  of  slaked  lime. 

71.  Quick  lime,  when  exposed  to  the  free  action  of  the  air  in 
a  dry  locality,  slakes  slowly,  by  imbibing  moisture  from  the  at- 
mosphere, with  a  slight  disengagement  of  heat.  Opinion  seems 
to  be  divided  with  regard  to  the  effect  of  this  method  of  slaking 
on  fat  limes.  Some  assert,  that  the  mortar  made  from  them  is 
better  than  that  obtained  from  any  other  process,  and  attribute 
this  result  to  the  re-conversion  of  a  portion  of  the  slaked  lime  into 
a  carbonate  ;  others  state  the  reverse  to  obtain,  and  assign  the 
same  cause  for  it.  With  regard  to  hydraulic  limes,  all  agree  that 
they  are  greatly  injured  by  air-slaking. 

72.  Air-slaked  fat  limes  increase  two  fifths  in  weight,  and  for 
one  volume  of  quick  lime  yield  3.52  volumes  of  slaked  lime.  The 
meager  limes  increase  one  eighth  in  weight,  and  for  one  volume 
of  quick  lime  yield  from  1.75  to  2.25  volumes  of  slaked  lime 

73.  The  dry  hydrates  of  lime,  when  exposed  to  the  atmosphere, 
gradually  absorb  carbonic  acid  and  water.  This  process  pro- 
ceeds very  slowly,  and  the  slaked  lime  never  regains  all  the  car- 
bonic acid  which  is  driven  off  by  the  calcination  of  the  lime-stone. 
When  converted  into  a  thick  paste,  and  exposed  to  the  air,  the 
hydrates  gradually  absorb  carbonic  acid  ;  this  action  first  takes 
place  on  the  surface,  and  proceeds  more  slowly  from  year  to 
year  towards  the  interior  of  the  exposed  mass.  The  absorption 
of  gas  pr oceeds  more  rapidly  in  the  meager  than  in  the  fat  limes. 
Those  hydrates  which  are  most  thoroughly  slaked  become  hard 
est.     The  hydrates  of  the  pure  fat  limes  become  in  time  very 


28  BUILDING  MATERIALS. 

hard,  while  those  of  the  hydraulic  limes  become  only  moderately 
hard. 

74.  The  fat  limes,  when  slaked  by  drowning,  may  be  pre- 
served for  a  long  >eriod  in  the  state  of  paste,  if  placed  in  a  damp 
situation  and  kepi  from  contact  with  the  air.  They  may  also  be 
preserved  for  a  long  time  without  change,  when  slaked  by  im- 
mersion to  a  dry  powder,  if  placed  in  covered  vessels.  Hydraulic 
limes,  under  similar  circumstances,  will  harden  if  kept  in  the  state 
of  paste,  and  will  deteriorate  when  in  powder,  unless  kept  in 
perfectly  air-tight  vessels. 

75.  The  hydrates  of  fat  lime,  from  air-slaking  or  immersion, 
require  a  smaller  quantity  of  water  to  reduce  them  to  the  state  of 
paste  than  the  others  ;  but,  when  immersed  in  water,  they  grad- 
ually imbibe  their  full  dose  of  water,  the  paste  becoming  thicker, 
but  remaining  unchanged  in  volume.  'Exposed  in  this  way,  the 
water  will  in  time  dissolve  out  all  the  lime  of  the  hydrate  which 
has  not  been  re-converted  into  a  sub-carbonate,  by  the  absorption 
of  carbonic  acid  before  immersion  ;  and  if  the  water  contain  car- 
bonic acid,  it  will  also  dissolve  the  carbonated  portions. 

76.  The  hydrates  of  hydraulic  lime,  when  immersed  in  water 
in  the  state  of  thin  pastes,  reject  a  portion  of  the  water  from  the 
paste,  and  become  hard  in  time  ;  if  the  paste  be  very  stiff,  they 
imbibe  more  water,  set  quickly,  and  acquire  greater  hardness  in 
time  than  the  soft  pastes.  The  pastes  of  the  hydrates  of  hydrau- 
lic lime,  which  have  hardened  in  the  air,  will  retain  their  hardness 
when  placed  in  water. 

77.  The  pastes  of  the  fat  limes  shrink  very  unequally  in  drying, 
and  the  shrinkage  increases  with  the  purity  of  the  lime  ;  on  this 
account  it  is  difficult  to  apply  them  alone  to  any  building  purposes, 
except  in  very  thin  layers.  The  pastes  of  the  hydraulic  limes 
can  only  be  used  with  advantage  under  water,  or  where  they  arc 
constantly  exposed  to  humidity ;  and  in  these  situations  they  are 
never  used  alone,  as  they  are  found  to  succeed  as  well,  and  to 
present  more  economy,  when  mixed  with  a  portion  of  sand 

78.  Manner  of  Reducing  Hijdraulic  Cement.  As  the  cement 
stones  will  not  slake,  they  must  be  reduced  to  a  fine  powder  by 
some  mechanical  process,  before  they  can  be  converted  into  a 
hydrate.  The  methods  usually  employed  for  this  purpose  con- 
sist in  first  breaking  the  burnt  stone  into  small  fragments,  either 
under  iron  cylinders,  or  in  mills  suitably  formed  for  this  pur- 
pose, which  are  next  ground  between  a  pair  of  stones,  or  else 
crushed  by  an  iron  roller.  The  coarser  particles  are  separated 
from  the  fine  powder  by  the  ordinary  processes  with  sieves.  The 
powder  is  then  carefully  packed  in  air-tight  casks,  and  kept  for  use 

79.  Hydraulic  cement,  like  hydraulic  lime,  deteriorates  by 
exoosurr  to  the  air,  and  may  in  time  lose  all  its  hydraulic  prop. 


LIMK.  29 

erties.  On  this  account  it  should  be  used  when  fresh  from  the 
kiln  ;  for,  however  carefully  packed,  it  cannot  be  well  preserved 
when  transported  to  any  distance. 

80.  The  deterioration  of  hydraulic  cements,  from  exposure  to 
the  air,  arises,  probably,  from  a  chemical  disunion  between  the 
constituent  elements  of  the  burnt  stone,  occasioned  by  the  ab- 
sorption of  water  and  carbonic  acid.  When  injured,  their  energy 
can  be  restored  by  submitting  them  to  a  much  slighter  degree  of 
heat  than  that  which  is  requisite  to  calcine  the  stone  suitably  in 
the  first  instance.  From  the  experiments  of  M.  Petot,  it  appears 
that  a  red  heat,  kept  up  for  a  short  period,  is  sufficient  to  restore 
damaged  hydraulic  cements. 

81.  Artificial  Hydraulic  Limes  and  Cements .  The  discovery 
of  the  argillaceous  character  of  the  stones  which  yield  hydraulic 
limes  and  cements,  connected  with  the  fact  that  brick  reduced  to 
a  fine  powder,  as  well  as  several  substances  of  volcanic  origin 
having  nearly  the  same  constituent  elements  as  ordinary  brick 
when  mixed  in  suitable  proportions  with  common  lime,  will  yield 
a  paste  that  hardens  under  water,  has  led,  within  a  recent  period, 
to  artificial  methods  of  producing  compounds  possessing  the  prop- 
erties of  natural  hydraulic  lime-stones. 

82.  M.  Vicat  was  the  first  to  point  out  the  method  of  forming 
an  artificial  hydraulic  lime,  by  mixing  common  lime  and  unburnt 
clay,  in  suitable  proportions,  and  then  calcining  them.  The  ex- 
periments of  M.  Vicat  have  been  repeated  by  several  eminent 
engineers  with  complete  success,  and  among  others  by  General 
Pasley,  who,  in  a  recent  work  by  him,  Observations  on  Limes, 
Calcareous  Cements,  &c,  has  given,  with  minute  detail,  the  results 
of  his  experiments  ;  from  which  it  appears  that  an  hydraulic  ce- 
ment, fully  equal  in  quality  to  that  obtained  from  natural  stones 
can  be  made  by  mixing  common  lime,  cither  in  the  state  of  a 
carbonate  or  of  a  hydrate,  with  clay,  and  subjecting  the  mixture 
to  a  suitable  degree  of  heat.  In  some  parts  of  France,  where 
chalk  is  found  abundantly,  the  preparation  of  artificial  hydraulic 
lime  has  become  a  branch  of  manufacture. 

83.  Different  methods  have  been  pursued  in  preparing  this 
material,  the  main  object  being  to  secure  the  finest  mechanical 
division  of  the  two  ingredients,  and  their  thorough  mixture.  For 
this  purpose  the  lime-stone,  if  soft  like  chalk  or  tufa,  may  be  re- 
duced in  a  wash-mill,  or  a  rolling-mill,  to  the  state  of  a  soft  pulp; 
it  is  then  incorporated  with  the  clay,  by  passing  them  through  a 
pug-mill.  The  mixture  is  next  moulded  into  small  blocks,  or 
made  up  into  balls  between  2  and  3  inches  diameter,  by  hand, 
and  \\  ell  dried.  The  balls  are  placed  in  a  kiln, — suitably  calcined, 
and  are  finally  slaked,  or  ground  down  fine  for  use. 

84.  If  the  lime-stone  be  hard,  it  must  be  calcined  and  slaked 


30 


BUILDING  MATERIALS. 


in  the  usual  m  '.nner,  before  it  can  be  mixed  witn  the  clay.  The 
process  for  m.xing  the  ingredients,  their  calcination,  and  farthei 
preparation  for  use,  are  the  same  as  in  the  preceding  case. 

85.  Artificial  hydraulic  lime,  prepared  from  the  hard  lime- 
stones, is  more  expensive  than  that  made  from  the  soft ;  but  it  is 
stated  to  be  superior  in  quality  to  the  latter. 

86.  As  clays  are  seldom  free  from  carbonate  of  lime,  and  as 
the  lime-stones  which  yield  common  or  fat  lime  may  contain  some 
portion  of  clay,  the  proper  proportions  of  the  two  ingredients,  to 
produce  either  an  hydraulic  lime  or  a  cement,  must  badetermmed 
by  experiment  in  each  case,  guided  by  a  previous  analysis  of  the 
two  ingredients  to  be  tried. 

If  the  lime  be  pure,  and  the  clay  be  free  from  lime,  then  the 
combinations  in  the  proportions  given  in  the  table  of  M.  Petot  will 
give,  by  calcination,  like  results  with  the  same  proportions  when 
found  naturally  combined. 

87.  Puzzolana,  &c.  The  practice  of  using  brick  or  tile-dust, 
or  a  volcanic  substance  known  by  the  name  of  puzzolana,  mixed 
with  common  lime,  to  form  an  hydraulic  lime,  was  known  to  the 
Romans,  by  whom  mortars  composed  of  these  materials  were 
extensively  used  in  their  hydraulic  constructions.  This  practice 
has  been  more  or  less  followed  by  modern  engineers,  who,  until 
within  a  few  years,  either  used  the  puzzolana  of  Italy,  where  it 
is  obtained  near  Mount  Vesuvius,  in  a  pulverulent  state,  or  a  ma- 
terial termed  Trass,  manufactured  in  Holland,  by  grinding  to  a  fine 
powder  a  volcanic  stone  obtained  near  Andernach  on  the  Rhine. 

Experiments  by  several  eminent  chemists  have  extended  the 
list  of  natural  substances  which,  when  properly  burnt  and  reduced 
to  powder,  have  the  same  properties  as  puzzolana.  They  mostly 
belong  to  the  feldspathic  and  schistose  rocks,  and  are  ejther  fine 
sand,  or  clays  more  or  less  indurated. 

The  following  Table  gives  the  results  of  analyses  of  Puzzolana, 
Trass,  a  Basalt,  and  a  Schistus,  tohich,  when  burnt  and  pow~ 
dered,  were  found  to  possess  the  properties  of  puzzolana. 


Puzzolana. 

Trass. 

Basalt. 

Schistus. 

Silica 

0.445 

0.570 

44.50 

46.00 

Alumina 

u 

0.150 

0.120 

16.75 

26.00 

Lime 

g 

0.088 

0.026 

9.50 

4.00 

Magnesia 

. 

0.047 

0.010 

- 

- 

Oxide  of  iron 

0.120 

0.050 

20.00 

14.00 

Oxide  of  manganese 

- 

- 

2.37 

8.00 

Potassa 

,        . 

0.014 

0.070 

- 

- 

Soda 

,        . 

0.030 

0.010 

2.60 

_ 

Water  and  loss 

• 

0.106 

0.144 

4.28 

200 

1.000 

1.000 

100.00 

100.00 

I.TME.  31 

88.  All  of  the?  J  substances,  when  prepared  artificially,  are  no\» 
generally  known  by  the  name  of  artificial  puzzolanas,  in  contra- 
distinction to  those  which  occur  naturally. 

89.  General  Treussart,  of  the  French  Corps  of  Military  Engi- 
neers, first  attempted  a  systematic  investigation  of  the  properties 
of  artificial  puzzolanas  made  from  ordinary  clay,  and  of  the  best 
manner  of  preparing  them  on  a  large  scale.  It  appears  from  the 
results  of  his  experiments,  that  the  plastic  clays  used  for  tiles,  or 
pottery,  which  are  unctuous  to  the  touch,  the  alumina  in  them 
being  in  the  proportion  of  one  fifth  to  one  third  of  the  silica,  fur- 
nish the  best  artificial  puzzolanas  when  suitably  burned.  The 
clays  which  are  more  meager,  and  harsher  to  the  touch,  yield  an 
inferior  article,  but  are  in  some  cases  preferable,  from  the  greater 
ease  with  which  they  can  be  reduced  to  a  powder. 

90.  As  the  clays  mostly  contain  lime,  magnesia,  some  of  the 
metallic  oxides,  and  alkaline  salts,  General  Treussart  endeavored 
to  ascertain  the  influence  of  these  substances  upon  the  qualities  of 
the  artificial  puzzolanas  from  clays  in  which  they  are  found.  He 
states,  ihat  the  carbonate  of  potash  and  the  muriate  of  soda  seem 
to  act  beneficially ;  that  magnesia  seems  to  be  passive,  as  well 
as  the  oxide  of  iron,  except  when  the  latter  is  found  in  a  large 
proportion,  when  it  acts  hurtfully ;  and  that  the  lime  has  a  mate- 
rial influence  on  the  degree  of  heat  required  to  convert  the  clay 
into  a  good  artificial  puzzolana. 

91.  The  management  of  the  heat,  in  the  preparation  of  this 
material,  seems  of  the  first  consequence  ;  and  General  Treussart 
recommends  that,  direct  experiment  be  resorted  to,  as  the  most 
certain  means  of  ascertaining  the  proper  point.  For  this  purpose, 
specimens  of  the  clay  to  be  tried  may  be  kneaded  into  balls  as 
large  as  an  egg,  and  the  balls,  when  dry,  be  submitted  to  different 
degrees  of  heat  in  a  kiln,  or  furnace,  through  which  a  current  of 
air  must  pass  over  the  balls,  as  this  last  circumstance  is  essential 
to  secure  a  material  possessing  the  best  hydraulic  qualities.  Some 
of  the  balls  are  withdrawn  as  soon  as  their  color  indicates  that 
they  are  underburnt ;  others  when  they  have  the  appearance  of 
well-burnt  brick ;  and  others  when  their  color  shows  that  they 
are  overburnt,  but  before  they  become  vitrified.  The  burnt  balls 
are  reduced  to  an  impalpable  powder,  and  this  is  mixed  with  a 
hydrate  of  fat  lime,  in  the  proportion  of  two  parts  of  the  powder 
t(  :>ne  of  lime  in  paste.  Water  is  added,  if  necessary,  to  bring 
the  different  mixtures  to  the  consistence  of  a  thick  pulp ;  and  they 
are  separately  placed  in  glass  vessels,  covered  with  water,  and 
allowed  to  remain  until  they  harden.  The  compound  which 
hardens  most  promptly  will  indicate  the  most  suitable  degree  of 
heat  to  be  applied. 

92.  As  the    arbonates  of  line,  of  potash,  and  of  soda,  act  as 


32  BUILDING  MATERIALS. 

fluxes  on  silica,  the  presence  ot  either  one  of  them  will  modify 
the  degree  of  heat  necessary  to  convert  the  clay  into  a  good  natu- 
ral puzzolana.  Clay,  containing  about  one  tenth  of  lime,  should 
be  brought  to  about  the  state  of  slightly-burnt  brick.  The  ochreous 
clays  require  a  higher  degree  of  heat  to  convert  them  into  a  good 
material,  and  should  be  burnt  until  they  assume  the  appearance 
of  well-burnt  brick.  The  more  refractory  clays  will  bear  a  still 
higher  degree  of  heat ;  but  the  calcination  should  in  no  case  be 
carried  to  the  point  of  incipient  vitrification. 

93.  The  quantity  of  lime  contained  in  the  clay  can  be  readily 
ascertained  beforehand,  by  treating  a  small  portion  of  the  clay, 
diffused  in  water,  with  enough  muriatic  acid  to  dissolve  out  the 
lime  ;  and  this  last  might  serve  as  a  guide  in  the  preliminary 
stages  of  the  experiments. 

94.  General  Treussart  states,  as  the  results  of  his  experiments, 
that  the  mixture  of  artificial  puzzolana  and  fat  lime  forms  an  hy 
drauiic  paste  superior  in  quality  to  that  obtained  by  M.  Yicat's 
process  for  making  artificial  hydraulic  lime.  M.  Curtois,  a  French 
civil  engineer,  in  a  memoir  on  these  artificial  compounds,  pub- 
lished in  the  Annales  des  Ponts  et  Chaussees,  1834,  and  General 
Pasley,  more  recently,  adopt  the  conclusion  of  General  Treussart. 
M.  Vicat's  process  appears  best  adapted  when  chalk,  or  any  very 
soft  lime-stone,  which  can  be  readily  converted  to  a  soft  pulp,  is 
used,  as  offering  more  economy,  and  affording  an  hydraulic  lime 
which  is  sufficiently  strong  for  most  building  purposes.  By  it 
General  Pasley  has  succeeded  in  obtaining  an  artificial  hydraulic 
cement,  which  is  but  little,  if  at  all,  inferior  to  the  best  natural 
varieties ;  a  result  which  has  not  been  obtained  from  any  com- 
bination of  fat  lime  with  puzzolana,  whether  natural,  or  artificial. 

95.  All  the  puzzolanas  possess  the  important  property  of  not 
deteriorating  by  exposure  to  the  air,  which  is  not  the  case  witli 
any  of  the  hydraulic  limes,  or  cements.  This  property  may  ren- 
der them  very  serviceable  in  many  localities,  where  only  common, 
or  feebly  hydraulic  lime  can  be  obtained. 

MORTAR. 

96.  Mortar  is  any  mixture  of  lime  in  paste  with  sand.  It  may 
be  divided  into  two  principal  classes  ;  Hydraulic  mortar,  which  is 
made  of  hydraulic  lime,  and  Common  mortar,  made  of  common 
lime. 

97.  The  term  Grout  is  applied  to  any  mortar  in  a  thin  or  fluid 
state  ;  and  the  terms  Concrete  and  Beton,  to  mortars  incorporated 
with  gravel  and  small  fragments  of  stone  or  brick. 

98.  Mortar  is  used  for  various  purposes  in  building.  It  servea 
as  a  cement  to  unite  blocks  of  stone,  or  brick.     In  concrete  and 


MORTAR.  33 

beton,  which  may  be  regarded  as  artificial  conglomerate  stones 
it  forms  the  matrix  by  which  the  gravel  and  broken  stone  are 
held  logether ;  and  it  is  the  principal  material  with  which  the  ex 
terior  surfaces  of  walls  and  the  interior  of  edifices  are  coated. 

99.  The  quality  of  mortars,  whether  used  for  structures  ex- 
posed to  the  weather,  or  for  those  immersed  in  water,  will  depend 
upon  the  nature  of  the  materials  used  ; — their  proportion  ; — the 
manner  in  which  the  lime  has  been  converted  into  a  paste  to  re- 
ceive the  sand ; — and  the  mode  employed  to  mix  the  ingredients. 
Upon  all  of  these  points  experiment  is  the  only  unerring  guide  for 
the  engineer  ;  for  the  great  diversity  in  the  constituent  elements 
of  lime-stones,  as  well  as  in  the  other  ingredients  of  mortars,  must 
necessarily  alone  give  rise  to  diversities  in  results  ;  and  when,  to 
these  causes  of  variation,  are  superadded  those  resulting  from 
different  processes  pursued  in  the  manipulations  of  slaking  the 
lime  and  mixing  the  ingredients,  no  surprise  should  be  felt  at  the 
seemingly  opposite  conclusions  at  which  writers,  who  have  pur 
sued  the  subject  experimentally,  have  arrived.  From  the  great 
mass  of  facts,  however,  presented  on  this  subject  within  a  few 
years,  some  general  rules  may  be  laid  down,  which  the  engineer 
may  safely  follow,  in  the  absence  of  the  means  of  making  direct 
experiments. 

100.  Sand.  This  material,  which  forms  one  of  the  ingredients 
of  mortar,  is  the  granular  product,  arising  from  the  disintegration 
of  rocks.  It  may,  therefore,  like  the  rocks  from  which  it  is  de- 
rived, be  divided  into  three  principal  varieties — the  silicious,  the 
calcareous,  and  the  argillaceous. 

Sand  is  also  named  from  the  locality  where  it  is  obtained,  as 
pit-sand,  which,  is  procured  from  excavations  in  alluvial,  or  other 
deposites  of  disintegrated  rock  ;  river-sand  and  sea-sand,  which 
are  taken  from  the  shores  of  the  sea,  or  rivers. 

Builders  again  classify  sand  according  to  the  size  of  the  gram. 
The  term  coarse  sand  is  applied  when  the  grain  varies  between 
}th  and  TVth  of  an  inch  in  diameter ;  the  term  fine  sand,  when 
the  grain  is  between  T\ th  and  ^th  of  an  inch  in  diameter  ;  and 
the  term  mixed  sand  is  used  for  any  mixture  of  the  two  prece- 
ding kinds. 

101.  The  silicious  sands,  arising  from  the  quartzose  rocks,  are 
the  most  abundant,  and  are  usually  preferred  by  builders.  The 
calcareous  sands,  from  hard  calcareous  rocks,  are  more  rare,  but 
form  a  good  ingredient  for  mortar.  Some  of  the  argillaceous  sand* 
possess  the  properties  of  the  less  energetic  puzzolanas,  and  are 
therefore  very  valuable,  as  forming,  with  common  lime,  an  arti- 
ficial hydraulic  lime. 

102.  The  property  which  some  argillaceous  sands  possess,  of 
forming  with  common,  or  slightly  hydrauUc  lime  a  compound  which 

5 


34  BUILDING  MATERIALS. 

will  harden  under  water,  has  been  long  known  in  France,  where 
these  sands  are  termed  arenes.  The  sands  of  this  nature  are 
usually  found  in  hillocks  along  river  valleys.  These  hillocks 
sometimes  rest  on  calcareous  rocks,  or  argillaceous  tufas,  and  are 
frequently  formed  of  alternate  beds  of  the  sand  and  pebbles.  The 
sand  is  of  various  colors,  such  as  yellow,  red,  and  green,  and 
seems  to  have  been  formed  from  the  disintegration  of  clay  in  a 
more  or  less  indurated  state.  The  arenes  are  not  as  energetic  as 
either  natural  or  artificial  puzzolanas  ;  still  they  form,  with  com- 
mon lime,  an  excellent  mortar  for  masonry  exposed  either  to  the 
open  air,  or  to  humid  localities,  as  the  foundations  of  edifices. 

103.  Pit-sand  has  a  rougher  and  more  angular  grain  than  river 
or  sea  sand  ;  and,  on  this  account,  is  generally  preferred  by  build- 
ers for  mortar  used  for  brick,  or  stone-work.  Whether  it  forms  a 
stronger  mortar  than  the  other  two  is  not  positively  settled,  al- 
though some  experiments  would  lead  to  the  conclusion  that  it 
does. 

104.  River  and  sea  sand  are  by  some  preferred  for  plastering, 
because  they  are  whiter,  and  have  a  finer  and  more  uniform  grain 
than  pit  sand ;  but  as  the  sands  from  the  shores  of  tidal  waters 
contain  salts,  they  should  not  be  used,  owing  to  their  hygrqmetric 
properties,  before  the  salts  are  dissolved  out  in  fresh  water  by 
careful  washing. 

105.  Pit-sand  is  seldom  obtained  free  from  a  mixture  of  dirt, 
or  clay  ;  and  these,  when  found  in  any  notable  quantity  in  it,  give 
a  weak  and  bad  mortar.  Earthy  sands  should,  therefore,  be 
cleansed  from  dirt  before  using  them  for  mortar ;  this  may  be 
effected  by  washing  the  sand  in  shallow  vats,  and  allowing  the 
turbid  water,  in  which  the  clay,  dust,  and  other  like  impurities 
are  held  in  suspension,  to  run  off. 

106.  Sand,  when  pure  or  well  cleansed,  may  be  known  by  not 
soiling  the  fingers  when  rubbed  between  them. 

107.  Hydraulic  mortar.  This  material  may  be  made  from 
the  natural  hydraulic  limes ;  from  those  which  are  prepared  by 
M.  Vicat's  process  ;  or  from  a  mixture  of  common,  or  feebly  hy- 
draulic lime,  with  a  natural  or  artificial  puzzolana.  All  writers, 
however,  agree  that  it  is  better  to  use  a  natural  than  an  artificial 
hydraulic  lime,  when  the  former  can  be  readily  procured. 

108.  When  the  lime  used  is  strongly  hydraulic,  M.  Vicat  is 
of  opinion  that  sand  alone  should  be  used  with  it,  to  form  a  good 
hydraulic  mortar.  General  Treussart  has  drawn  the  conclusion, 
from  his  experiments,  that  the  mortar  of  all  hydraulic  limes  is 
improved  by  an  addition  of  a  natural  or  artificial  puzzolana.  The 
quantity  of  sand  used  may  vary  from  \{  to  2  parts  of  the  lime 
<in  bulk,  when  reduced  to  a  thick  pulp. 

109.  For  hydraulic  mortars,  made  of  common,  feeble,  or  or 


MQRTAR.  35 

dinary  hydraulic  limes,  and  artificial  puzzolana,  M.  Vicat  statei 
tbat  the  puzzolana  should  be  the  weaker  as  the  lime  is  more 
strongly  hydraulic  ;  using,  for  example,  a  very  energetic  puzzo- 
lana with  a  fat,  or  a  feebly  hydraulic  lime.  The  proportion  of 
sand  which  can  be  incorporated  with  these  ingredients,  to  form  an 
hydraulic  mortar,  is  stated  by  General  Treussart  to  be  one  vol- 
ume to  one  of  puzzolana,  and  one  of  lime  in  paste. 

110.  In  proportioning  the  ingredients,  the  object  to  which  the 
mortar  is  to  be  applied  should  be  regarded.  When  it  is  to  serve 
to  unite  stone,  or  brick  work,  it  is  better  that  the  hydraulic  lime 
should  be  rather  in  excess  :  when  it  is  used  as  a  matrix  for  beton, 
no  more  lime  should  be  used  than  is  strictly  required.  No  harm 
will  arise  from  an  excess  of  good  hydraulic  lime,  in  any  case  ;  but 
an  excess  of  common  lime  is  injurious  to  the  quality  of  the  mortar. 

\  1 1 .  Common  and  ordinary  hydraulic  limes,  when  made  into 
mortar  with  arenes,  give  a  good  material  for  hydraulic  purposes. 
The  proportions  in  which  these  have  been  found  to  succeed  well, 
are  one  of  lime  to  three  of  arenes. 

112.  Hydraulic  cement,  from  the  promptitude  with  which  it 
hardens,  both  in  the  air  and  under  water,  is  an  invaluable  mate- 
rial where  this  property  is  essential.  Any  dose  of  sand  injures 
its  properties  as  a  cement.  But  hydraulic  cement  may  be  added 
with  decided  advantage  to  a  mortar  of  common,  or  of  feebly  hy 
draulic  lime  and  sand.  It  is  in  this  way  that  it  is  generally  used 
in  our  public  works.  The  French  engineers  give  the  preference 
to  a  good  hydraulic  mortar  over  hydraulic  cement,  both  for  uniting 
stone,  or  brick  work,  and  for  plastering.  They  find,  from  their 
practice,  that  when  used  as  a  stucco,  it  does  not  withstand  well 
the  effects  of  weather ;  that  it  swells  and  cracks  in  time ;  and, 
when  laid  on  in  successive  coats,  that  they  become  detached  frum 
each  other. 

General  Pasley,  who  lias  paid  great  attention  to  the  properties 
of  natural  and  artificial  hydraulic  cements,  does  not  agree  with 
the  French  engineers  in  his  conclusions.  He  states  that,  when 
skilfully  applied,  hydraulic  cement  is  superior  to  any  hydraulic 
mortar  for  masonry,  but  that  it  must  be  used  only  in  thin  joints  : 
and,  when  applied  as  a  stucco,  that  it  should  be  laid  on  in  but  one 
coat ;  or,  if  it  be  laid  on  in  two,  the  second  must  be  added  long 
before  the  first  has  set,  so  that,  in  fact,  the  two  make  but  one 
coal.  By  attending  to  these  precautions,  General  Pasley  states 
that  a  stucco  of  hydraulic  cement  and  sand  will  wkns'and  per- 
fectly the  effects  of  frost. 

113.  Mortars  exposed  to  weather.  The  French  engineers, 
who  have  paid  great  attention  to  the  subject  of  mortars,  coincide 
in  the  opinion,  that  a  mortar  cannot  be  made  of  fat  lime  and  any 
inert  sands,  like  those  of  the  silicious,  or  calcareous  kinds   which 


86  BUILDING  MATERIALS. 

will  withstand  the  ordinary  exposure  of  weather  ;  and  that,  te 
obtain  a  good  mortar  for  this  purpose,  either  the  hv  ..Vaulic  limes 
mixed  with  sand  must  be  employed,  or  else  common  lime  mixed 
either  with  arenes,  or  with  a  puzzolana  and  sand. 

1 14.  Any  pure  sand  mixed  in  proper  proportions  with  hydraulic 
lime,  w ill  give  a  good  mortar  for  the  open  air;  but  the  hardness 
of  the  mortar  will  be  affected  by  the  size  of  the  grain,  particularly 
when  hydraulic  lime  is  used.  Fine  sand  yields  the  best  mortar 
with  good  hydraulic  lime  ;  mixed  sand  with  the  feebly  hydraulic 
limes  ;  and  coarse  sand  with  fat  lime. 

115.  The  proportion  which  the  lime  should  bear  to  the  sand 
seems  to  depend,  in  some  measure,  on  the  manner  in  which  the 
lime  is  slaked.  M.  Vicat  states,  that  the  strength  of  mortar  made 
of  a  stiff  paste  of  fat  lime,  slaked  in  the  ordinary  way,  increases 
from  0.50  to  2.40  to  one  of  the  paste  in  volume  ;  and  that,  when 
the  lime  is  slaked  by  immersion,  one  volume  of  the  like  paste  will 
give  a  mortar  that  increases  in  strength  from  0.50  to  2.20  parts 
of  sand. 

For  one  volume  of  a  paste  of  hydraulic  lime,  slaked  in  the  or- 
dinary way,  the  strength  of  the  mortar  increases  from  0  to  1 .80 
f>arts  of  sand  ;  and,  when  slaked  by  immersion,  the  morta*  of  a 
ike  paste  increases  in  strength  from  0  to  1.70  parts  of  lime.  In 
every  case,  when  the  dose  of  sand  was  increased  beyond  these 
proportions,  the  strength  of  the  resulting  mortar  was  found  to 
decrease. 

116.  Manipulations  of  Mortar.  The  quality  of  hydraulic  mor- 
tar, which  is  to  be  immersed  in  water,  is  more  affected  by  the 
manner  in  which  the  lime  is  slaked,  and  the  ingredients  mixed, 
than  that  of  mortar  which  is  to  be  exposed  to  the  weather ;  al- 
though in  both  cases  the  increase  of  strength,  by  the  best  manipu- 
lations, is  sufficient  to  make  a  study  of  them  a  matter  of  some 
consequence. 

117.  The  results  obtained  from  the  ordinary  method  of  slak- 
ing, by  sprinkling,  or  by  immersion,  in  the  case  of  good  hydraulic 
limes,  are  nearly  the  same.  Spontaneous,  or  air-slaking,  gives 
invariably  the  worst  results.  For  common  and  slightly  hydraulic 
lime,  M.  Vicat  states  that  air-slaking  yields  the  best  results,  and 
ordinary  slaking  the  worst. 

118.  The  ingredients  of  mortar  are  incorporated  either  by 
manual  labor,  or  by  machinery  :  the  latter  method  gives  results 
superior  to  the  former.  The  machines  commonly  used  for  mix- 
ing mortar  are  either  the  ordinary  pug-mill  (Fig.  4)  employed  by 
brickmakers  for  tempering  clay,  or  a  grinding-mill,  (Fig.  5.) 
The  grinding-mill  is  the  best  machine,  because  it  not  only  re- 
duces the  lumps,  which  are  found  in  the  most  carefully  burnt 
Btone,  after  the  slaking  is  apparently  complete,  but  it  brings  the 


MORTAR. 


37 


fime  lo  the  state  of  a  uniform  stiff  naste,  wh'ch  it  should  leceive 
before  the  sand  is  incorporated  w.th  it.     Care  should  be  taken 


El     D 


Fig.  4  represents  a  vertical  section  through 
the  axis  of  a  pug-mill,  for  mixing  of 
tempering  mortar.— This  mill  consists 
of  a  hooped  vessel,  of  the  form  of  a  co- 
nical frustum,  which  receives  the  in- 
gredients, and  a  vertical  shaft,  to  which 
arms  with  teeth,  resembling  an  ordi- 
nary rake,  are  attached,  for  the  purpose 
of  mixing  the  ingredients. 

A,  A,  section  of  sides  of  the  vessel. 

B,  vertical  shaft  to  which  the  arms  C  are 
affixed. 

D,  horizontal  bar  for  giving  a  circular  mo- 
tion to  the  shaft  B. 

E,  sills  of  timber  supporting  the  mill. 

F,  wrought-iron  support  through  which 
the  upper  part  of  the  shaft  passes. 


not  to  add  too  much  water,  particularly  when  the  mortar  is  to  be 
immersed  in  water.  The  mortar-mill,  on  this  account,  should  be 
sheltered  from  rain  ;  and  the  quantity  of  water  with  which  it  is 

Fig.  5  represents  a  part  of  a  mill  for  crushing  the 
lime  and  tempering  the  mortar. 

A,  heavy  wheel  of  timber,  or  cast  iron. 

B,  horizontal  bar  passing  through  the  wheel,  which 
at  one  extremity  is  fixed  to  a  vertical  shaft,  and 
is  arranged  at  the  other  (C)  with  the  proper  gear- 
ing for  a  horse. 

D,  a  circular  trough,  with  a  trapezoidal  cross  sec- 
tion which  receives  the  ingredients  to  be  mixed. 
The  trough  may  be  from  20  to  30  feet  in  diameter ; 
about  18  inches  wide  at  top,  and  12  inches  deep; 
and  be  built  of  hard  brick,  stone,  or  timber  laid  on 
a  firm  foundation. 

supplied  may  vary  with  the  state  of  the  weather.  Nothing  seems 
to  be  gained  by  carrying  the  process  of  mixing,  beyond  obtainin 
a  uniform  mass  of  the  consistence  of  plastic  clay.  Mortars  o 
hydraulic  lime  are  injured  by  long  exposure  to  the  air,  and  fre- 
quent turnings  and  mixings  with  a  shovel  or  spade  ;  those  of 
common  lime,  under  like  circumstances,  seem  to  be  improved. 
Mortar,  which  has  been  set  aside  for  a  day  or  two,  will  become 
sensibly  firmer ;  if  not  allowed  to  stand  too  long,  it  may  be  again 
reduced  to  its  clayey  consistence,  by  simply  pounding  it  with  a 
beetle,  without  any  fresh  addition  of  water. 

119.  Setting  and  Durability  of  Mortar s.  Mortar  of  common 
lime,  without  any  addition  of  puzzolana,  will  not.  set  in  humid 
situations,  like  the  foundations  of  edifices,  until  after  a  very  long 
lapse  of  time.  They  set  very  soon  when  exposed  to  the  air,  or 
jo  an  atmosphere  of  carbonic  acid  gas.     If,  after  having  become 


38  BUILDING  MATERIALS. 

liard  in  the  open  air,  they  are  placed  under  water,  ihey  in  tim* 
lose  their  cohesion  and  fall  to  pieces. 

120.  Common  mortars,  which  have  had  time  to  harden,  resist 
the  action  of  severe  frosts  very  well,  if  they  are  made  rather  poor, 
or  with  an  excess  of  sand.  The  sand  should  be  over  2.40  parts, 
in  bulk,  to  one  volume  of  the  lime  in  paste  ;  and  coarse  sand  is 
found  to  give  better  results  than  fine  sand. 

121.  Good  hydraulic  mortars  set  equally  well  in  damp  situa- 
tions, and  in  the  open  air ;  and  those  which  have  hardened  in  the 
air  will  retain  their  hardness  when  immersed  in  water.  They 
also  resist  well  the  action  of  frost,  if  they  have  had  time  to  set 
before  exposure  to  it ;  but,  like  common  mortars,  they  require  to 
be  made  with  an  excess  of  sand,  to  withstand  well  atmospheric 
changes. 

122.  The  surface  of  a  mass  of  hydraulic  mortar,  whether  made 
of  a  natural  hydraulic  lime  or  otherwise,  when  immersed  in  water, 
becomes  more  or  less  degraded  by  the  action  of  the  water  upon  the 
lime,  particularly  in  a  current.  When  the  water  is  stagnant,  a 
very  thin  crust  of  carbonate  of  lime  forms  on  the  surface  of  the 
mass,  owing  to  the  absorption  by  the  lime  of  the  carbonic  acid 
gas  in  the  water.  This  crust,  if  the  water  be  not  agitated,  will 
preserve  the  soft  mortar  beneath  it  from  the  farther  action  of  the 
water,  until  it  has  had  time  to  become  hard,  when  the  water  will 
no  longer  act  upon  the  lime  in  any  perceptible  degree. 

123.  Hydraulic  mortars  set  with  more  or  less  promptness,  ac- 
cording to  the  character  of  the  hydraulic  lime,  or  of  the  puzzolana 
which  enters  into  their  composition.  Artificial  hydraulic  mortars. 
with  an  excess  of  lime,  set  more  slowly  than  when  the  lime  is  in 
a  just  proportion  to  the  other  ingredients. 

124.  The  quick-setting  hydraulic  limes  are  said  to  furnish  a 
mortar  which,  in  time,  acquires  neither  as  much  strength  nor 
hardness  as  that  from  the  slower-setting  hydraulic  limes.  Ar- 
tificial hydraulic  mortars,  on  the  contrary,  which  set  quickly, 
gain,  in  time,  more  strength  and  hardness  than  those  which  set 
slowly. 

125.  The  time  in  which  hydraulic  mortars,  immersed  in  water, 
attain  their  greatest  hardness,  is  not  well  ascertained.  Mortars 
made  of  strong  hydraulic  limes  do  not  show  any  appreciable  in- 
crease of  hardness  after  the  second  year  of  their  immersion  ;  while 
the  best  artificial  hydraulic  mortars  continue  to  harden,  in  a  sen- 
sible degree,  during  the  third  year  after  their  immersion. 

126.  Theory  of  Mortars.  The  paste  of  a  hydrate,  either  of 
common  or  of  hydraulic  lime,  when  exposed  to  the  air,  absorbs 
carbonic  acid  gas  from  it ;  passes  to  the  state  of  sub-carbonate  of 
lime ;  without,  however,  rejecting  the  water  of  the  hydrate,  and 
gradually  hardens.     The  time  required  for  the  complete  satuta 


MORTAIi  3S 

ucn  of  the  mass  exposed,  will  depend  on  its  bulk.  The  absorp- 
tion of  the  gas  commences  at  the  surface  and  proceeds  more 
slowly  towards  the  centre.  The  hardening  of  mortars  exposed 
to  the  atmosphere,  is  generally  attributed  to  this  absorption  of  the 
gas,  as  no  chemical  action  of  lime  upon  quartzose  sand,  which  is 
the  usual  kind  employed  for  mortars,  has  hitherto  been  detected 
by  the  most  careful  experiments. 

1 27.  With  regard  to  hydraulic  mortars,  it  is  difficult  to  account 
for  their  hardening,  except  upon  the  effect  which  the  silicate  of 
lime  may  have  upon  the  excess  of  simple  hydrate  of  uncombined 
lime  contained  in  the  mass.  M.  Petot  supposes,  that  the  parti- 
cles of  silicate  of  lime  form  so  many  centres,  around  which  the 
uncombined  hydrates  group  themselves  in  a  crystalline  form ; 
becoming  thus  sufficiently  hard  to  resist  the  solvent  action  of 
water.  With  respect  to  the  action  of  quartzose  sand  in  hydraulic 
mortars,  M.  Petot  thinks  that  the  grains  produce  the  same  me- 
chanical effect  as  the  particles  of  the  silicate  of  lime,  in  inducing 
the  aggregation  of  the  uncombined  hydrate. 

128.  Concrete.  This  term  is  applied,  by  English  architects 
and  engineers,  to  a  mortar  of  finely-pulverized  quick-lime,  sand, 
and  gravel.  These  materials  are  first  thoroughly  mixed  in  a  dry 
state,  sufficient  water  is  added  to  bring  the  mass  to  the  ordinary 
consistence  of  mortar,  and  it  is  then  rapidly  worked  up  by  a 
shovel,  or  else  passed  through  a  pug-mill.  The  concrete  is  used 
immediately  after  the  materials  arc  well  incorporated,  and  while 
the  mass  is  hot. 

129.  The  materials  for  concrete  are  compounded  in  various 
proportions.  The  most  approved  are  those  in  which  the  limo 
and  sand  are  in  the  proper  proportions  to  form  a  good  mortar 
and  the  gravel  is  twice  the  bulk  of  the  sand.  The  gravel  used 
should  be  clean,  and  any  pebbles  contained  in  it  larger  than 
an  egg,  should  be  broken  up  before  the  materials  are  incorpo- 
rated. 

1 30.  Hot  water  has  in  some  cases  been  used  in  making  con 
crete.     It  causes  the  mass  to  set  more  rapidly,  but  is  not  other- 
wise of  any  advantage. 

131.  The  bulk  of  a  mass  of  concrete,  when  first  made,  is  found 
to  be  about  one  fifth  less  than  the  total  bulk  of  the  dry  materials 
But,  as  the  lime  slakes,  the  mass  of  concrete  is  found  to  expand 
about  three  eighths  of  an  inch  in  height,  for  every  foot  of  the  mass 
in  depth. 

132.  The  usn.  of  concrete  is  at  present  mostly  restricted  to 
forming  a  solid  bed,  in  bad  soils,  for  the  foundations  of  edifices. 
It  has  also  been  used  to  form  blocks  of  artificial  stone,  for  the 
walls  of  buildings  and  other  like  purposes  ;  but  experience  has 
shown,  that  i*  possesses  neither  the  durability  nor  strength  requj 


40  BUILDING  MATERIALS. 

site  for  structures  of  a  permanent  character,  when  exposed  to  the 
action  of  water,  or  of  the  weather. 

133.  Beton.  The  term  beton  is  applied,  by  French  engineers, 
to  any  mixture  of  hydraulic  mortar  with  fragments  of  brick,  stone, 
or  gravel ;  and  it  is  now  also  used  by  English  engineers  in  the 
same  sense. 

134.  The  proportions  of  the  ingredients  used  for  beton  are  va- 
riously stated  by  different  authors.  The  sole  object  for  which 
the  gravel,  or  the  broken  stone  is  used,  being  to  obtain  a  more 
economical  material  than  a  like  mass  of  hydraulic  mortar  alone 
would  yield,  the  quantity  of  broken  stone  should  be  as  great  as 
can  be  thoroughly  united  by  the  mortar.  The  smallest  amount  of 
mortar,  therefore,  that  can  be  used  for  this  purpose,  will  be  that 
which  will  be  just  equal  in  volume  to  the  void  spaces  in  any  given 
bulk  of  the  broken  stone,  or  gravel.  The  proportion  which  the 
volume  occupied  by  the  void  spaces  bears  to  any  bulk  of  a  loose 
material,  like  broken  stone,  or  gravel,  may  be  readily  ascertained 
by  filling  a  vessel  of  known  capacity  with  the  loose  material,  and 
pouring  in  as  much  water  as  the  vessel  will  contain.  The  vol- 
ume of  water  thus  found,  will  be  the  same  as  that  of  the  void 
spaces. 

1 35.  Beton  made  of  mortar  and  broken  stone,  in  which  the 
proportions  of  the  ingredients  were  ascertained  by  the  process 
just  detailed,  has  been  found  to  give  satisfactory  results  ;  but,  in 
order  to  obviate  any  defect  arising  from  imperfect  manipulation, 
it  is  usual  to  add  an  excess  of  mortar  above  that  of  the  void 
spaces. 

The  best  and  most  economical  beton  is  made  of  a  mixture  of 
broken  stone,  or  brick,  in  fragments  not  larger  than  a  hen's  egg, 
and  of  coarse  and  fine  gravel  mixed  in  suitable  proportions. 

136.  In  making  beton,  the  mortar  is  first  prepared,  and  then 
incorporated  with  the  finer  gravel ;  the  resulting  mixture  is  spread 
out  into  a  cake,  4  or  6  inches  in  thickness,  over  which  the  coarser 
gravel  and  broken  stone  are  uniformly  strewed  and  pressed  down, 
the  whole  mass  being  finally  brought  to  a  homogeneous  state  with 
the  hoe  and  shovel. 

Beton  is  used  for  the  same  purposes  as  concrete,  to  which  it 
is  superior  in  every  respect,  but  particularly  so  for  foundations 
laid  under  water,  or  in  humid  localities. 

1 37.  Adherence  of  Mortar.  The  force  with  which  mortars  in 
general  adhere  to  other  materials,  depends  on  the  nature  of  the 
material,  its  texture,  and  the  state  of  the  surface  to  which  the 
mortar  is  applied. 

138.  Mortar  adheres  most  strongly  to  brick  ;  and  more  feebly 
o  wood  than  to  any  other  material.     Among  stones,  its  adhesion 

to  lime-stone  is  generally  greatest ;  and  to  basalt  and  sand-stones 


MASTICS.  4] 

'cast .  Among  stones  of  the  same  class,  it  adheres  generally  bet- 
ter o  the  porous  and  coarse-grained,  than  to  the  compact  and 
fine-grained.  Among  surfaces,  it  adheres  more  strongly  to  the 
rough  than  to  the  smooth. 

139.  The  adhesion  of  common  mortar  to  brick  and  stone,  for 
the  first  few  years,  is  greater  than  the  cohesion  of  its  own  parti- 
cles. The  force  with  which  hydraulic  cement  adheres  to  the  same 
materials,  is  less  than  that  of  the  cohesion  between  its  own  parti- 
cles :  and.  from  some  recent  experiments  of  Colonel  Pasley,  on 
this  subject,  it  would  seem  that  hydraulic  cement  adheres  with 
nearly  the  same  force  to  polished  surfaces  of  stone  as  to  rough 
surfaces. 

140.  From  experiments  made  by  Rondelet,  on  the  adhesion  of 
common  mortar  to  stone,  it  appears  that  it  required  a  force  vary- 
ing from  15  to  30  pounds  on  the  square  inch,  applied  perpendicu- 
lar to  the  plane  of  the  joint,  to  separate  the  mortar  and  stone 
after  six  months  union ;  whereas,  only  5  pounds  to  the  square 
inch  was  required  to  separate  the  same  surfaces,  when  applied 
parallel  to  the  plane  of  the  joint. 

From  experiments  made  by  Colonel  Pasley,  he  concludes  that 
the  adhesive  force  of  hydraulic  cement  to  stone,  may  be  taken  as 
high  as  125  pounds  on  the  square  inch,  when  the  joint  has  had 
time  to  harden  throughout ;  but,  he  remarks,  that  as  in  large 
joints  the  exterior  part  of  the  joint  may  have  hardened  while  the 
interior  still  remains  soft,  it  is  not  safe  to  estimate  the  adhesive 
force,  in  such  cases,  higher  than  from  30  to  40  pounds  on  the 
square  inch. 

MASTICS. 

141.  The  term  Mastic  is  generally  applied  to  artificial  or  natu 
ral  combinations  of  bituminous  or  resinous  substances  with  other 
ingredients.  They  are  converted  to  various  uses  in  constructions, 
either  as  cements  for  other  materials,  or  as  coatings,  to  render  them 
impervious  to  water. 

142.  Bituminous  Mastic.  The  knowledge  of  this  material 
dates  back  to  an  early  period ;  but  it  is  only  within,  compara- 
tively speaking,  a  few  years  that  it  has  come  into  common  use  in 
Europe  and  this  country.  The  most  usual  form  in  which  it  if 
now  employed,  is  a  combination  of  mineral  tar  and  powdered 
bituminous  lime-stone. 

143.  The  localities  of  each  of  these  substances  are  very  nu- 
merous ;  but  they  are  chiefly  brought  into  the  market  from  several 
places  in  Switzerland  and  France,  where  these  mirerals  are  found 
in  great  abundance  ;  the  most  noted  being  Val-de-Travers  in 
Switzerland,  and  Seyssel  in  France. 

144.  The  mineral  tar  is  usually  obtained  by  bo'ling  in  water  a 

6 


42  BUILDING  MATERIALS. 

uoft  sand-stone,  called  by  the  French  molasse,  which  is  stiongly 
impregnated  with  the  tar.  In  this  process,  the  tar  is  disengaged 
and  rises  to  the  surface  of  the  water,  or  adheres  to  the  sides  of 
the  vessel,  and  the  earthy  matter  remains  at  the  bottom.  An 
analysis  of  a  rich  specimen  of  the  Seyssel  bituminous  sand-stone 
gave  the  following  results  : — 

Bituminous  oil         .     .086  >  Bitumen 
Carbon  .         .         .     .020 ) 

Quartzy  grains 690 

Calcareous  grains  ......     .204 


1.000 


145.  The  bituminous  lime-stone  which,  when  reduced  to  a 

f)owdered  state,  is  mixed  with  the  mineral  tar,  is  known  at  the 
ocalities  mentioned  by  the  name  of  asphaltum,  an  appellation 
which  is  now  usually  given  to  the  mastic.  This  lime-stone  occurs 
in  the  secondary  formations,  and  is  found  to  contain  various  pro- 
portions of  bitumen,  varying  mostly  from  3  to  15  per  cent.,  with 
the  other  ordinary  minerals,  as  argile,  &c,  which  are  met  with 
in  this  formation. 

146.  The  bituminous  mastic  is  prepared  from  these  two  mate- 
rials by  heating  the  mineral  tar  in  cast-iron  or  sheet-iron  boilers, 
and  stirring  in  the  proper  proportion  of  the  powdered  lime- 
stone. This  operation,  although  very  simple  in  its  kind,  requires 
great  attention  and  skill  on  the  part  of  the  workmen  in  managing 
the  fire,  as  the  mastic  may  be  injured  by  too  low,  or  too  high  a 
degree  of  heat.  The  best  plan  appears  to  be,  to  apply  a  brisk 
fii  3  until  the  boiling  liquid  commences  to  give  out  a  thin  whitish 
vapor.  The  fire  is  then  moderated  and  kept  at  a  uniform  state, 
and  the  powdered  stone  is  gradually  added,  and  mixed  in  with  the 
tar  by  stirring  the  two  well  together.  When  the  temperature  has 
been  raised  too  high,  the  heated  mass  gives  out  a  yellowish  or 
brownish  vapor.  In  this  state  it  should  be  stirred  rapidly,  and  be 
removed  at  once  from  the  fire. 

147.  The  asphaltic  stone  may  be  reduced  to  powder,  either  by 
roasting  it  in  vessels  over  a  fire,  or  by  grinding  it  down  in  the  or- 
dinary mortar-mill.  For  roasting,  the  stone  is  first  reduced  to 
fragments  the  size  of  an  egg.     These  fragments  are  put  into  an 

ion  vessel ;  heat  is  applied,  and  the  stone  is  reduced  to  powder 
by  stirring  it  and  breaking  it  up  with  an  iron  instrument.  This 
process  is  not  only  less  economical  than  grinding,  but  the  ma- 
terial loses  a  portion  of  its  tar  from  evaporation,  besides  being 
liable  to  injury  from  too  great  a  degree  of  heat.  For  grinding 
the  stone  is  first  broken  as  for  roasting.  Care  should  be  taken, 
during  the  process,  to  stir  the  mass  frequently,  otherwise  it  may 


GLUE.  43 

form  into  a  cake.  Cold  d'-y  weather  is  the  best  season  for  this 
operation ;  the  stone,  however,  should  not  be  exposed  to  the 
weather. 

148.  Owing  to  the  variable  quantity  of  mineral  tar  in  bitumi 
nous  lime-stone,  the  best  proportions  of  the  tar  and  powdered  stone 
for  bituminous  mastic,  cannot  be  assigned  beforehand.  Three  01 
four  per  cent,  too  much  of  tar,  is  said  to  impair  both  the  durability 
and  tenacity  of  the  mastic ;  while  too  small  a  quantity  is  equally 
prejudicial.  Generally,  from  eight  to  ten  per  cent,  of  the  tar,  by 
weight,  has  been  found  to  yield  a  favorable  result. 

149.  Mastics  have  been  formed  by  mixing  vegetable  tar,  pitch, 
and  other  resinous  substances,  with  litharge,  powdered  brick, 
powdered  lime-stone,  &c. ;  but  the  results  obtained  have  gener- 
ally been  inferior  to  those  from  bituminous  mastic. 

150.  Mineral  tar  is  more  durable  than  vegetable  tar,  and  on  this 
account  it  has  been  used  alone  as  a  coating  for  other  materials, 
but  not  with  the  same  success  as  mastic.  Employed  in  this  way, 
the  tar  in  time  becomes  dry  and  peels  off;  whereas,  in  the  form 
of  mastic,  the  hard  matter  with  which  it  is  mixed  prevents  the 
evaporation  of  the  oily  portion  of  the  tar,  and  thus  promotes  its 
durability. 

151.  The  uses  to  which  bituminous  mastic  is  applied  are  daily 
increasing.  It  has  been  used  for  paving  in  a  variety  of  forms, 
either  as  a  cement  for  large  blocks  of  stone,  or  as  the  matrix  of  a 
concrete  formed  of  small  fragments  of  stone  or  gravel ;  as  a  point- 
ing, it  is  found  to  be  more  serviceable,  for  some  purposes,  than 

1 1  ilic  cement ;  it  forms  one  of  the  best  water-tight  coatings 
for  cisterns,  cellars,  the  cappings  of  arches,  terraces,  and  other 
similar  roofings  now  in  use  ;  and  is  a  good  preservative  agent  for 
wood  work  exposed  to  wet  or  damp. 

GLUE. 

1 52.  The  common  animal  glue  is  seldom  used  as  a  cement  for 
any  other  purpose  than  for  the  work  of  the  joiner.  Although  of 
considerable  tenacity,  it  is  weak,  brittle,  and  readily  impaired  by 
moisture. 

1 53.  Within  a  few  years  back,  a  material  termed  marine  glue, 
the  invention  of  Mr.  Jeffery  of  England,  has  attracted  attention  in 
England  and  France,  in  both  which  countries  its  qualities  as  a 
cement,  both  for  stone  and  wood,  have  been  tested  with  the  most 
satisfactory  results.  This  composition  is  said  to  be  made  by  first 
dissolving  caoutchouc  in  coal  naphtha,  in  the  proportion  of  one 
pound  of  the  former  to  five  gallons  of  the  latter  ;  to  this  solution 
an  equal  weight  of  shellac  is  added,  and  the  composition  is  then 
placed  over  a  fire  and  thoroughly  mixed  by  stirring. 


44  BUILDING  MATERIALS. 

154.  Owing  to  its  insolubility  in  water,  its  remarkable  tenacity 
and  adhesion,  and  its  powers  of  contraction  and  expansion  through 
a  very  considerable  range  of  temperature,  without  becoming  either 
very  soft  or  brittle,  the  marine  glue  promises  to  be  not  only  a  val 
uable  addition  to  the  resources  of  the  naval  architect,  but  to  the 
civil  engineer. 

BRICK. 

155.  This  material  is  properly  an  artificial  stone,  formed  by 
submitting  common  clay,  which  has  undergone  suitable  prepara- 
tion, to  a  temperature  sufficient  to  convert  it  into  a  semi-vitrified 
state. 

156.  Brick  may  be  used  for  nearly  all  the  purposes  to  which 
stone  is  applicable  ;  for  when  carefully  made,  its  strength,  hard 
ness,  and  durability,  are  but  little  inferior  to  the  more  ordinary 
kinds  of  building  stone.  It  remains  unchanged  under  the  ex- 
tremes of  temperature  ;  resists  the  action  of  water  ;  sets  firmly 
and  promptly  with  mortar ;  and  being  both  cheaper  and  lighter 
than  stone,  is  preferable  to  it  for  many  kinds  of  structures,  as 
arches,  the  walls  of  houses,  &c. 

157.  The  art  of  brick-making  is  a  distinct  branch  of  the  useful 
arts,  and  does  not  properly  belong  to  that  of  the  engineer.     But 
as  the  engineer  is  frequently  obliged  to  prepare  this  material  him 
self,  the  following  outline  of  the  process  may  prove  of  service. 

158.  The  best  brick  earth  is  composed  of  a  mixture  of  pure 
clay  and  sand,  deprived  of  pebbles  of  every  kind,  but  particularly 
of  those  which  contain  lime,  and  pyritous,  or  other  metallic  sub- 
stances ;  as  these  substances,  when  in  large  quantities,  and  in  the 
form  of  pebbles,  act  as  fluxes,  and  destroy  the  shape  of  the  brick, 
and  weaken  it  by  causing  cavities  and  cracks  ;  but  in  small  quan- 
tities, and  equally  diffused  throughout  the  earth,  they  assist  the 
vitrification,  and  give  it  a  more  uniform  character. 

159.  Good  brick  earth  is  frequently  found  in  a  natural  state, 
and  requires  no  other  preparation  for  the  purposes  of  the  brick- 
maker.  When  he  is  obliged  to  prepare  the  earth  by  mixing  the 
pure  clay  and  sand,  direct  experiments  should,  in  all  cases,  be 
made,  to  ascertain  the  proper  proportions  of  the  two.  If  the  clay 
is  in  excess,  the  temperature  required  to  semi-vitrify  it,  will  cause 
it  to  warp,  shrink,  and  crack  ;  and,  if  there  is  an  excess  of  sand, 
complete  vitrification  will  ensue,  under  similar  circumstances. 

160.  The  quality  of  the  brick  depends  as  much  on  the  care 
bes'owed  on  its  manufacture,  as  on  the  quality  of  the  earth.  The 
firs  stage  of  the  process  is  to  free  the  earth  from  pebbles,  which 
is  most  effectually  done  by  digging  it  out  early  in  the  autumn, 
and  exposing  it  in  small  heaps  to  the  weather  during  the  winter. 
In  the  spring,  the  heaps  are  carefully  riddled,  if  necessary,  and 


BRICK.  43 

the  earth  ia  then  in  a  proper  state  to  be  kneaded  or  tempered. 

The  quantity  of  water  required  in  tempering,  will  depend  on  the 
quality  of  the  earth ;  no  more  should  be  used,  than  will  be  suffi 
cient  to  make  the  earth  so  plastic,  as  to  admit  of  its  being  easily 
moulded  by  the  workman.  About  half  a  cubic  foot  of  water  tc 
one  of  the  earth  is,  in  most  cases,  a  good  proportion.  If  too  much 
water  be  used,  the  brick  will  not  only  be  very  slow  in  drying,  bu. 
it  will,  in  most  cases,  crack,  owing  to  the  surface  becoming  com- 
pletely dry,  before  the  moisture  of  the  interior  has  had  time  to 
escape  ;  the  consequence  of  which  will  be,  that  the  brick,  when 
burnt,  will  be  either  entirely  unfit  for  use,  or  very  weak. 

161.  Machinery  is  now  coming  into  very  general  use  in  mould- 
ing brick  :  it  is  superior  to  manual  labor,  not  only  from  the  labor 
saved,  but  from  its  yielding  a  better  quality  of  brick,  by  giving  it 
great  density,  which  adds  to  its  strength. 

]()2.  Great  attention  is  requisite  in  drying  the  brick  before  it 
burned.  It  should  be  placed,  for  this  purpose,  in  a  dry  expo- 
sure, and  be  sheltered  from  the  direct  action  of  the  wind  and  sun, 
in  order  that  the  moisture  may  be  carried  off  slowly  and  uniformly 
from  the  entire  surface.  When  this  precaution  is  not  taken,  the 
brick  will  generally  crack  from  the  unequal  shrinking,  arising 
from  one  part  drying  more  rapidly  than  the  rest. 

163.  The  burning  and  cooling  should  be  done  with  equal  care. 
A  very  moderate  fire  should  be  applied  under  the  arches  of  the 
kiln  for  about  twenty-four  hours,  to  expel  any  remaining  moisture 
from  the  raw  brick ;  this  is  known  to  be  completely  effected, 
when  the  smoke  from  the  kiln  is  no  longer  black.  The  fire  is 
then  increased  until  the  bricks  of  the  arches  attain  a  white  heat; 
it  is  then  allowed  to  abate  in  some  degree,  in  order  to  prevent 
complete  vitrification  ;  and  it  is  alternately  raised  and  lowered  in 
this  way,  until  the  burning  is  complete,  which  may  be  ascer- 
tained by  examining  the  bricks  at  the  top  of  the  kiln.  The 
cooling  should  be  slowly  effected ;  otherwise  the  bricks  will  not 
withstand  the  effects  of  the  weather.  It  is  done  by  closing 
the  mouths  of  the  arches,  and  the  top  and  sides  of  the  kiln  in 
the  most  effectual  manner  with  moist  clay  and  burnt  brick,  and 
allowing  the  kiln  to  remain  in  this  state  until  the  warmth  has 
subsided. 

164.  Brick  of  a  good  quality  exhibits  a  fine,  compact,  uniform 
texture,  when  broken  across  ;  gives  a  clear  ringing  sound,  when 
struck  ;  and  is  of  a  cherry  red,  or  brownish  color.  Three  varie- 
ties are  found  in  the  kiln  ;  those  which  form  the  arches,  denom 
mated  arch  brick,  are  always  vitrified  in  part,  and  present  a 
grayish  glassy  appearance  at  one  end  ;  they  are  very  hard,  but 
brittle,  of  inferior  strength,  and  set  badly  with  mortar ;  those  from 
the  interior  of  the  kiln,  usually  denominated  body,  hard,  or  cherry 


46  BUILDING  MATERIALS. 

brick,  are  of  the  best  quality ;  those  from  near  the  top  and  sides, 
are  generally  underburnt,  and  are  denominated  soft,  pale,  or  sam 
mel  brick ;  they  have  neither  sufficient  strength,  nor  durability 
for  heavy  masonry,  nor  the  outside  courses  of  walls,  which  are 
exposed  to  the  weather. 

1 65.  The  quality  of  good  brick  may  be  improved  by  soaking 
it  for  some  days  in  water,  and  re-burning  it.  This  process  in- 
creases both  the  strength  and  durability,  and  renders  the  brick 
more  suitable  for  hydraulic  constructions,  as  it  is  found  not  to 
imbibe  water  so  readily  after  having  undergone  it. 

166.  The  size  and  form  of  bricks  present  but  trifling  variations. 
They  are  generally  rectangular  parallelopipeds,  from  eight  to  nine 
»nches  long,  from  four  to  four  and  a  half  wide,  and  from  two  to 
iwo  and  a  quarter  thick.  Thin  brick  is  generally  of  a  better 
quality  than  thick,  because  it  can  be  dried  and  burned  more 
uniformly. 

167.  Fire-brick.  This  material  is  used  for  the  facing  of  fur- 
naces, fireplaces,  &c,  where  a  high  degree  of  temperature  is  to  be 
sustained.  It  is  made  of  a  very  refractory  kind  of  pure  clay,  that 
remains  unchanged  by  a  degree  of  heat  which  would  vitrify  and  com- 
pletely destroy  ordinary  brick.  A  very  remarkable  brick  of  this 
character  has  been  made  of  agaric  mineral;  it  remains  un 
changed  under  the  highest  temperature,  is  one  of  the  worst  con- 
ductors of  heat,  and  so  light  that  it  will  float  on  water. 

168.  Tiles.  As  a  roof  covering,  tiles  are,  in  many  respects, 
superior  to  slate,  or  metallic  coverings.  They  are  strong  and 
durable,  and  are  very  suitable  for  the  covering  of  arches,  as  their 
great  weight  is  not  so  objectionable  here,  as  in  the  case  of  roofs 
formed  of  frames  of  timber. 

Tiles  should  be  made  of  the  best  potter's  clay,  and  be  moulded 
with  great  care  to  give  them  the  greatest  density  and  strength. 
They  are  of  very  variable  form  and  size  ;  the  worst  being  the 
flat  square  form,  as,  from  the  liability  of  the  clay  to  warp  in  burn- 
ing, they  do  not  make  a  perfectly  water-tight  covering. 

WOOD. 

169.  This  material  holds  the  next  rank  to  stone,  owing  to  its 
durability  and  strength,  and  the  very  gener  tl  use  made  of  it  in 
constructions.  To  suit  it  to  the  purposes  of  the  engineer,  the 
free  is  felled  after  having  attained  its  mature  growth,  and  the 
trunk,  the  larger  branches  that  spring  from  the  trunk,  and  the 
main  parts  of  the  root,  are  cut  into  suitable  dimensions,  and  sea- 
soned, in  which  state,  the  term  timber  is  applied  to  it.  The 
crooked,  or  compass  timber  of  the  branches  and  roots,  is  mostly 
applied  to  the  purooses  of  ship-building,  for  the  knees  and  other 


WOOD.  4? 

parts  of  the  frame-work  of  vessels,  requi/mg  crooked  timber 
The  Hunk  furnishes  all  the  straight  timber. 

170.  The  trunk  of  a  full-grown  tree  presents  three  distind 
parts  :  the  bark,  which  forms  the  exterior  coating ;  the  sap-vjood, 
which  is  next  to  the  bark  ;  the  heart,  or  inner  part,  which  is  easily 
distinguishable  from  the  sap-wood  by  its  greater  firmness  and 
darker  color. 

171.  The  heart  forms  the  essential  part  of  the  trunk,  as  a 
building  material.  The  sap-wood  possesses  but  little  strength, 
and  is  subject  to  rapid  decay,  owing  to  the  great  quantity  of  fer- 
mentable matter  contained  in  it ;  and  the  bark  is  not  only  without 
strength,  but,  if  suffered  to  remain  on  the  tree  after  it  is  felled,  it 
hastens  the  decay  of  the  sap-wood  and  heart. 

172.  Trees  should  not  be  felled  for  timber  until  they  have  at- 
tained their  mature  growth,  nor  after  they  exhibit  symptoms  of 
decline  ;  otherwise,  the  timber  will  be  less  strong,  and  far  less 
durable.  Most  forest  trees  arrive  at  maturity  between  fifty  and 
one  hundred  years,  and  commence  to  decline  after  one  hundred 
and  fifty  or  two  hundred  years.  The  age  of  the  tree  can,  in  most 
cases,  be  ascertained  either  by  its  external  appearances,  or  by 
cutting  into  the  centre  of  the  trunk,  and  counting  the  rings,  or 
layers  of  the  sap  and  heart,  as  a  new  ring  is  formed  each  year  in 
the  process  of  vegetation.  When  the  tree  commences  to  decline, 
the  extremities  of  the  old  branches,  and  particularly  the  top,  ex- 
hibit signs  of  decay. 

173.  Trees  should  not  be  felled  while  the  sap  is  in  circulation  ; 
for  this  substance  is  of  a  peculiarly  fermentable  nature,  and,  there- 
fore, very  productive  of  destruction  to  the  wood.  The  winter 
months,  and  July,  are  the  seasons  in  which  trees  are  felled  for 
timber,  as  the  sap  is  generally  considered  as  dormant  during  these 
months ;  this  practice,  however,  is  in  part  condemned  by  some 
writers  ;  and  the  recent  experiments  of  M.  Boucherie,  in  France, 
support  this  opinion,  and  indicate  midsummer  and  autumn  as  the 
seasons  in  which  the  sap  is  least  active,  and  therefore  as  most 
favorable  for  felling. 

174.  As  the  sap-wuod,  in  most  trees,  forms  a  large  portion  of 
the  trunk,  experiments  have  been  made,  for  the  purpose  of  im- 
proving its  strength  and  durability.  These  experiments  have  been 
mostly  directed  towards  the  manner  of  preparing  the  tree,  before 
felling  it.  One  method  consists  jn  girdling,  or  making  an  in 
cision  with  an  axe  around  the  trunk,  completely  through  the  sap- 
wood,  and  suffering  the  tree  to  stand  in  this  state  until  it  is  dead  ; 
the  other  consists  in  barking,  or  stripping  the  entire  trunk  of  its 
bark,  without  wounding  the  sap-wood,  early  in  the  spring,  and  al- 
lowing the  tree  to  stand  until  the  new  leaves  have  put  forth  and 
fallen,  before  it  is  felled.    The  sap-wood  of  trees,  treated  by  both 


48  BUILDING  MATERIALS. 

of  these  methods,  was  found  very  much  improved  in  hardness 
strength,  and  durability  ;  the  results  from  girdling  were,  however 
inferior  to  those  from  barking. 

175.  The  seasoning  of  timber  is  of  the  greatest  importance,  not 
only  to  its  durability,  but  to  the  solidity  of  the  structure  for  which 
it  may  be  used ;  as  a  very  slight  shrinking  of  some  of  the  pieces, 
arising  from  the  seasoning  of  the  wood,  might,  in  many  cases, 
cause  material  injury,  if  not  complete  destruction  to  the  structure. 
Timber  is  considered  as  sufficiently  seasoned,  for  the  purposes 
of  frame-work,  when  it  has  lost  abou.  one  fifth  of  the  weight 
which  it  has  in  a  green  state.  Several  methods  are  in  use  for 
seasoning  timber :  they  consist  either  in  an  exposure  to  the  air 
for  a  certain  period  in  a  sheltered  position,  which  is  termed  natu- 
ral seasoning ;  in  immersion  in  water,  termed  water  seasoning ; 
or  in  boiling,  or  steaming. 

176.  For  natural  seasoning,  it  is  usually  recommended  to  strip 
the  trunk  of  its  branches  and  bark,  immediately  upon  felling,  and 
to  remove  it  to  some  dry  position,  until  it  can  be  sawed  into  suit- 
able scantling.  From  the  experiments  of  M.  Boucherie,  just 
cited,  it  would  seem  that  better  results  would  ensue,  from  allow- 
ing the  branches  and  bark  to  remain  on  the  trunk  for  some  days 
after  felling.  In  this  state,  the  vital  action  of  the  tree  continuing 
in  operation,  the  sap-vessels  will  be  gradually  exhausted  of  sap, 
and  filled  with  air,  and  the  trunk  thus  better  prepared  for  the  pro- 
cess of  seasoning.  To  complete  the  seasoning,  the  sawed  timber 
shoujd  be  piled  under  drying  sheds,  where  it  will  be  freely  ex- 
posed to  the  circulation  of  the  air,  but  sheltered  from  the  direct 
action  of  the  wind,  rain,  and  sun.  By  taking  these  precautions, 
an  equable  evaporation  of  the  moisture  will  take  place  over  the 
entire  surface,  which  will  prevent  either  warping  or  splitting, 
which  necessarily  ensues  when  one  part  dries  more  rapidly  than 
another.  It  is  farther  recommended,  instead  of  piling  the  pieces 
on  each  other  in  a  horizontal  position,  that  they  be  laid  on  cast- 
iron  supports  properly  prepared,  and  with  a  sufficient  inclination 
to  facilitate  the  dripping  of  the  sap  from  one  end  ;  and  that  heavy 
round  timber  be  bored  through  the  centre,  to  expose  a  greater 
surface  to  the  air,  as  it  has  been  found  that  it  cracks  mrre  in  sea- 
soning than  square  timber. 

Natural  seasoning  is  preferable  to  any  other,  as  timber  seasoned 
in  this  way  is  both  stronger  and  more  durable  than  when  prepared 
by  any  artificial  process.  Most  timber  will  require,  on  an  aver- 
age, about  two  years  to  become  fully  seasoned  in  the  natural 
way. 

177.  The  process  of  seasoning  by  immersion  in  water,  is  slow 
and  imperfect,  as  it  takes  years  to  saturate  heavy  timber ;  and 
the  soluble  matter  is  dischargee1  very  slowly,  and  chiefly  fi  om  the 


wood.  49 

exterior  layers  of  the  immersed  wood.  The  practice  of  keeping 
timber  in  water,  with  a  view  to  facilitate  its  seasoning,  has  been 
condemned  as  of  doubtful  utility ;  particularly  immersion  in  salt 
water,  where  the  timber  is  liable  to  the  inroads  of  those  two  very 
destructive  inhabitants  of  our  waters,  the  Limnoria  Terebrans, 
and  Teredo  Navalis ;  the  former  of  which  rapidly  destroys  the 
heaviest  logs,  by  gradually  eating  in  between  the  annual  rings ; 
and  the  latter,  the  well-known  ship-worm,  by  converting  timber 
into  a  perfect  honeycomb  state  by  its  numerous  perforations. 

178.  Steaming  is  mostly  in  use  for  ship-building,  where  it  is 
necessary  to  soften  the  fibres,  for  the  purpose  of  bending  large 
pieces  of  timber.  This  is  effected  by  placing  the  timber  in  strong 
steam-tight  cylinders,  where  it  is  subjected  to  the  action  of  steam 
long  enough  for  the  object  in  view  ;  the  period  usually  allowed, 
is  one  hour  to  each  inch  in  thickness.  Steaming  slightly  impairs 
the  strength  of  timber,  but  renders  it  less  subject  to  decay,  and 
less  liable  to  warp  and  crack. 

179.  When  timber  is  used  for  posts  partly  imbedded  in  the 
ground,  it  is  usual  to  char  the  part  imbedded,  to  preserve  it  from 
decay.  This  method  is  only  serviceable  when  the  timber  has  been 
previously  well  seasoned  ;  but  for  green  timber  it  is  highly  inju- 
rious, as  by  closing  the  pores,  it  prevents  the  evaporation  from  the 
surface,  and  thus  causes  fermentation  and  rapid  decay  within. 

180.  The  most  durable  timber  is  procured  from  trees  of  a  close 
compact  texture,  which,  on  analysis,  yield  the  largest  quantity  of 
carbon.  And  those  which  grow  in  moist  and  shady  localities, 
furnish  timber  which  is  weaker  and  less  durable  than  that  from 
trees  growing  in  a  dry  open  exposure. 

181.  Timber  is  subject  to  defects,  arising  either  from  some 
peculiarity  in  the  growth  of  the  tree,  or  from  the  effects  of  the 
weather.  Straight-grained  timber,  free  from  knots,  is  superior 
in  strength  and  quality.,  as  a  building  material,  to  that  which  is 
the  reverse. 

182.  The  action  of  high  winds,  or  of  severe  frosts,  injures  the 
tree  while  standing  :  the  former  separating  the  layers  from  each 
other,  forming  what  is  denominated  rolled  timber;  the  latter 
cracking  the  timber  in  several  places,  from  thr  surface  to  th« 
centre.  These  defects,  as  well  as  those  arising  from  worms,  or 
age,  are  easily  seen  by  examining  a  cross  section  of  the  trunk. 

183.  The  wet  and  dry  rot  are  the  most  serious  causes  of  the 
decay  of  timber ;  as  all  the  remedies  thus  far  proposed  to  prevent 
them,  are  too  expensive  to  admit  of  a  very  general  application. 
Both  of  these  causes  have  the  same  origin,  fermentation,  and 
consequent  putrefaction.  The  wet  rot  takes  place  in  wood  ex- 
posed, alternately,  to  moisture  and  dryness ;  and  the  dry  rot  is 
occasioned  by  want  of  a  free  circulation  of  air,  as  in  confined 

7 


50  BUILDING  MATERIALS. 

warm  localities,  like  cellars  and  the  more  confined  parts  of 
vessels. 

Trees  of  rapid  growth,  which  contain  a  arge  portion  of  sap- 
wood,  and  timber  of  every  description,  wher.  used  green,  where 
there  is  a  want  of  a  free  circulation  of  air,  decay  very  rapidly  with 
the  rot. 

184.  Numberless  experiments  have  been  made  on  the  preser- 
vation of  timber,  and  many  processes  for  this  purpose  have  been 
patented  both  in  Europe  and  this  country.  Several  of  these 
processes  have  yielded  the  most  satisfactory  results ;  and  nearly 
all  have  proved  more  or  less  efficacious.  The  means  mostly  re- 
sorted to  have  been  the  saturation  of  the  timber  in  the  solution 
of  some  salt  with  a  metallic,  or  earthy  base,  thus  forming  an  in- 
soluble compound  with  the  soluble  matter  of  the  timber.  The 
salts  which  have  been  most  generally  tried,  are  the  sulphate  of 
iron,  or  copper,  and  the  chloride  of  mercury,  zinc,  or  calcium. 
The  results  obtained  from  the  chlorides  have  been  more  satisfac- 
tory than  those  from  the  sulphates  ;  the  latter  class  of  salts  with 
metallic  bases  possess  undoubted  antiseptic  properties  ;  but  it  is 
stated  that  the  freed  sulphuric  acid,  arising  from  the  chemical 
action  of  the  salt  on  the  wood,  impairs  the  woody  fibre,  and 
changes  it  into  a  substance  resembling  carbon. 

185.  The  processes  which  have  come  into  most  general  use, 
are  those  of  Mr.  Kyan,  and  of  Sir  W.  Burnett,  called  after  the 
patentees  kyanizing  and  burnetizing.  Kyan's  process  is  to  sat- 
urate the  timber  with  a  solution  of  chloride  of  mercury ;  using, 
for  the  solution,  one  pound  of  the  salt  to  five  gallons  of  water 
Burnett  uses  a  solution  of  chloride  of  zinc,  in  the  proportion  of 
one  pound  of  the  salt  to  ten  gallons  of  water,  for  common  pur- 
poses ;  and  a  more  highly  concentrated  solution  when  the  object 
is  also  to  render  the  wood  incombustible. 

186.  As  timber  under  the  ordinary  circumstances  of  immer- 
sion imbibes  the  solutions  very  slowly,  a  more  expeditious,  as 
well  as  more  perfect  means  of  saturation  has  been  used  of  late, 
which  consists  in  placing  the  wood  to  be  prepared  in  strong 
wrought-iron  cylinders,  lined  with  felt  and  boards,  to  protect  the 
iron  from  the  action  of  the  solution,  where,  first  by  exhausting 
the  cylinders  of  air,  and  then  applying  a  strong  pressure  by  means 
of  a  force-pump,  the  liquid  is  forced  into  the  sap  and  air-vessels, 
and  penetrates  to  the  very  centre  of  the  timber. 

187.  Among  the  patented  processes  in  our  country,  that  of  Mr. 
Earle  has  received  most  notice.  This  consists  in  boiling  the 
timber  in  a  solution  of  the  sulphates  of  copper  and  iron.  Opinion 
seems  to  be  divided  as  to  the  efficacy  of  this  method.  It  has  been 
tried  for  the  preservation  of  timber  for  artillery  carriages,  but  not 
with  satisfactory  results 


WOOD.  51 

188.  M.  Boucherie.  ,  whose  able  researches  on  this  subject 
reference  has  been  made,  noticing  the  slowness  with  which 
aqueous  solutions  were  imbibed  by  wood,  when  simply  im- 
mersed in  them,  conceived  the  ingenious  idea  of  rendering  the 
vital  action  of  the  sap-vessels  subservient  to  a  thorough  impreg- 
nation of  every  part  of  the  trunk  where  there  was  this  vitality 
To  effect  this,  he  first  immersed  the  butt  end  of  a  freshly-felled 
tree  in  a  liquid,  and  found  that  it  was  diffused  throughout  all  parts 
of  the  tree,  in  a  few  days,  by  the  action  in  question.  But,  find 
»ng  it  difficult  to  manage  trees  of  some  size  when  felled,  M. 
Boucherie  next  attempted  to  saturate  them  before  felling ;  for 
which  purpose  he  bored  an  auger-hole  through  the  trunk,  and 
made  a  saw-cut  from  the  auger-hole  outwards,  on  each  side,  to 
within  a  few  inches  of  the  exterior,  leaving  enough  of  the  fibres 
untouched  to  support  the  tree.  One  end  of  the  auger-hole  was 
then  stopped,  as  well  as  all  of  the  saw-cut  on  the  exterior,  and 
the  liquid  was  introduced  by  a  tube  inserted  into  the  open  end  of 
the  auger-hole.  This  method  was  found  equally  efficacious  with 
the  first,  and  more  convenient. 

189.  After  examining  the  action  of  the  various  neutral  salts  on 
the  soluble  matter  contained  in  wood,  M.  Boucherie  was  led  to 
try  the  impure  pyrolignite  of  iron,  both  from  its  chemical  compo- 
sition and  its  cheapness.  The  results  of  this  experiment  were 
perfectly  satisfactory.  The  pyrolignite  of  iron,  in  the  proportion 
of  one  fiftieth  in  weight  of  the  green  wood,  was  found  not  only  to 
preserve  the  wood  from  decay,  but  to  harden  it  to  a  very  high 
degree. 

190.  Observing  that  the  pliability  and  elasticity  of  wood  de- 
pended, in  a  great  measure,  on  the  moisture  contained  in  it,  M. 
Boucherie  next  directed  his  attention  to  the  means  of  improving 
these  properties.  For  this  purpose,  he  tried  solutions  of  various 
deliquescent  salts,  which  were  found  to  answer  the  end  proposed. 
Among  these  solutions,  he  gives  the  preference  to  that  of  chloride 
of  calcium,  which  also,  when  concentrated,  renders  the  wood  in-, 
combustible.  lie  also  recommends  for  like  purposes  the  mother 
water  of  salt-marshes,  as  cheaper  than  the  solution  of  the  chloride 
of  calcium.  Timber  prepared  in  this  way  is  not  only  improved 
in  elasticity  and  pliability,  but  is  prevented  from  warping  and 
cracking ;  the  timber,  however,  is  subject  to  greater  variations  in 
weight  than  when  seasoned  naturally. 

191.  M.  Boucherie  is  of  opinion  that  the  earthy  chlorides  will 
also  act  as  preservatives,  but  to  ensure  this  he  recommends  that 
they  be  mixed  with  one  fifth  of  pyrolignite  of  iron. 

192.  From  other  experiments  of  M.  Boucherie,  it  appears  thai 
the  sap  may  be  expelled  from  any  freshly-felled  timber  by  the 
pressure  of  a  liquid,  and  the  timber  be  impregnated  as  thoroughly 


52  BUILDING  MATERIALS. 

as  by  the  preceding  processes.  To  effect  this,  the  piece  to  be 
saturated  is  placed  in  an  upright  position,  so  that  the  sap  may 
flow  readily  from  the  lower  end  ;  a  water-tight  bag,  containing 
ihe  liquid,  is  affixed  to  the  upper  extremity  which  is  surmounted 
by  the  liquid,  the  pressure  from  which  expels  the  sap,  and  fills 
th  •  sap-vessels  with  the  liquid.  The  process  is  complete  when 
the  liquid  is  found  to  issue  in  a  pure  state  from  the  lower  end  of 
the  stick. 

193.  Either  of  the  above  processes  may  be  applied  in  impreg- 
nating timber  with  coloring  matter  for  ornamental  purposes.  The 
plan  recommended  by  M.  Boucherie,  consists  in  introducing  sep- 
arately the  solutions  by  the  chemical  union  of  which  the  color  is 
lo  be  formed. 

194.  The  effect  of  time  on  the  durability  of  timber,  prepared 
by  any  of  the  various  chemical  processes  which  have  just  been 
detailed,  remains  to  be  seen ;  although  results  of  the  most  satis- 
factory nature  may  be  looked  for,  considering  the  severe  tests  to 
which  most  of  them  have  been  submitted,  by  exposure  in  situa- 
tions peculiarly  favorable  to  the  destruction  of  ligneous  sub- 
stances. 

195.  The  durability  of  timber,  when  not  prepared  by  any  of 
the  above-mentioned  processes,  varies  greatly  under  different  cir- 
cumstances of  exposure.  If  placed  in  a  sheltered  position,  and 
exposed  to  a  lree  circulation  of  air,  timber  will  last  for  centuries, 
without  showing  any  sensible  changes  in  its  physical  proper- 
ties. An  equal,  if  not  superior,  durability  is  observed  when  it 
is  immersed  in  fresh  water,  or  embedded  in  thick  walls,  or 
under  ground,  so  as  to  be  beyond  the  influence  of  atmospheric 
changes. 

196.  In  salt  water,  however,  particularly  in  warm  climates, 
timber  is  rapidly  destroyed  by  the  two  animals  already  noticed  : 
the  one,  the  limnoria  terebrans,  attacking,  it  is  said,  only  station- 
ary wood,  while  the  attacks  of  the  other,  the  teredo  navalis,  are 
general.  Various  means  have  been  tried  to  guard  against  the 
ravages  of  these  destructive  agents ;  that  of  sheathing  exposed 
timber  with  copper,  or  with  a  coating  of  hydraulic  cement,  affixed 
to  the  wood  by  studding  it  thickly  over  with  broad-headed  nails  to 
give  a  hold  to  the  cement,  has  met  with  full  success  ;  but  the  oxi- 
dation of  the  metal,  and  the  liability  to  accident  of  the  cement, 
limit  their  efficacy  to  cases  where  they  can  be  renewed.  The 
chemical  processes  for  preserving  timber  from  decay,  do  not  ap- 
pear to  guard  them  in  salt  water.  A  process,  however,  of  pre- 
serving timber  by  impregnating  it  with  coal  tar,  patented  in  tiiis 
country  by  Professor  Renwick,  appears,  from  careful  experi- 
ments, also  to  be  efficacious  against  the  attack  of  the  snip-worm. 
A  coating  of  Jeffery's  marine  glue,  when  impregnated  with"  some 


wood.  53 

jf  the  insol  lble  mineral  poisons  destructive  to  animal  life,  is  said 
.o  subserve  the  same  end. 

197.  The  best  seasoned  timber  wiL  r  ot  withstand  the  effects 
of  exposure  to  the  weather  for  a  much  greater  period  than  twenty 
five  years,  unless  it  is  protected  by  a  coating  of  paint  or  pitch, 
or  of  oil  laid  on  hot,  when  the  timber  is  partly  charred  over  a  light 
blaze.  These  substances  themselves,  being  of  a  perishable  na 
ture,  require  to  be  renewed,  from  time  tu  time,  and  will,  there- 
fore, be  serviceable  only  in  situations  which  admit  of  their  renewal. 
They  are,  moreover,  more  hurtful  than  serviceable,  to  unseasoned 
timber,  as  by  closing  the  pores  of  the  exterior  surface,  they  pre- 
vent the  moisture  from  escaping  from  within,  and,  therefore,  pro- 
mote one  of  the  chief  causes  of  decay. 

198.  The  forests  of  our  own  country  produce  a  great  variety 
of  the  best  timber  for  every  purpose,  and  supply  abundantly  both 
our  own  and  foreign  markets.  The  following  genera  are  in  most 
common  use. 

199.  Oak.  About  forty-four  species  of  this  tree  are  enumera- 
ted by  botanists,  as  found  in  our  forests,  and  those  of  Mexico. 
The  most  of  them  afford  a  good  building  material,  except  the 
varieties  of  red  oak,  the  timber  of  which  is  weak,  and  decays 
rapidly. 

The  White  Oak,  (Quercus  Alba,)  so  named  from  the  color 
of  its  bark,  is  among  the  most  valuable  of  the  species,  and  is  in 
very  general  use,  but  is  mostly  reserved  for  naval  constructions  ; 
its  trunk,  which  is  large,  serving  for  heavy  frame-work,  and  the 
roots  and  larger  branches  affording  the  best  compass  timber.  The 
wood  is  strong  and  durable,  and  of  a  slightly  reddish  tinge  ;  it  is 
not  suitable  for  boards,  as  it  shrinks  about  fc  in  seasoning,  and 
is  very  subject  to  warp  and  crack. 

This  tree  is  found  most  abundantly  in  the  Middle  States.  It 
is  seldom  seen,  in  comparison  with  other  forest  trees,  in  the 
Eastern  and  Southern  States,  or  in  the  rich  valleys  of  the  West- 
ern States. 

Post  Oak,  (Quercus  Obtusiloba.)  This  tree  seldom  attains  a 
greater  diameter  than  about  fifteen  inches,  and,  on  this  account, 
is.  mostly  used  for  posts,  from  which  use  it  takes  its  name.  The 
wood  has  a  yellowish  hue,  and  close  grain ;  is  said  to  exceed 
white  oak  in  strength  and  durability ;  and  is,  therefore,  an  excel- 
lent building  material  for  the  lighter  kinds  of  frame-work.  This 
tree  is  found  most  abundantly  in  the  forests  of  Maryland  and  Vir- 
ginia, and  is  there  frequently  called  Box  White  Oak,  and  Iron 
Oak.  It  also  grows  in  the  forests  of  the  Southern  and  Western 
States,  but  is  rarely  seen  farther  north  than  the  mouth  of  the 
Hudson  River. 

Chesnut  White  Oak,  (Quercus  Primes  Palustris.)     The  tiir 


64  BUILDING  MATERIALS. 

oer  of  this  tree  i  strong  and  durable,  but  inferior  to  the  two  pre 
ceding  species.  The  tree  is  abundant  from  North  Carolina  U 
Florida. 

Rock  Chesnu  Oak,  (Quercus  Prinus  Monticola.)  The  timber 
of  this  tree  is  mostly  in  use  for  naval  constructions,  for  which  it 
is  esteemed  inferior  only  to  the  white  oak.  The  tree  is  fcund  in 
the  Middle  States,  and  as  far  north  as  Vermont. 

Live  Oak,  {Quercus  Virens.)  The  wood  of  this  tree  is  of  a 
yellowish  tinge ;  it  is  heavy,  compact,  and  of  a  fine  grain  ;  it  is 
stronger  and  more  durable  than  any  other  species,  and,  on  this 
account,  it  is  considered  invaluable  for  the  purposes  of  ship- 
building, for  whicli  it  is  exclusively  reserved. 

The  live  oak  is  not  found  farther  north  than  the  neighborhood 
of  Norfolk,  Virginia,  nor  farther  inland,  than  from  fifteen  to  twenty 
miles  from  the  seacoast.  It  is  found  in  abundance  along  the 
coast  south,  and  in  the  adjacent  islands  as  far  as  the  mouth  of  the 
Mississippi. 

v  200.  Pine.  This  very  interesting  genus  is  considered  inferior 
only  to  the  oak,  from  the  excellent  timber  afforded  by  nearly  all 
of  its  species.  It  is  regarded  as  a  most  valuable  building  mate 
rial,  owing  to  its  strength  and  durability,  the  straightness  of  its 
fibre,  the  ease  with  which  it  is  wrought,  and  its  applicability  to 
all  the  purposes  of  constructions  in  wood. 

Yellow  Pine,  (Pinus  Mitis.)  The  heart-wood  of  this  tree  is 
fine-grained,  moderately  resinous,  strong,  and  durable  ;  but  the 
sap-wood  is  very  inferior,  decaying  rapidly  on  exposure  to  the 
weather.    The  timber  is  in  very  general  use  for  frame-work,  &c. 

This  tree  is  found  throughout  our  country,  but  in  the  greatest 
abundance  in  the  Middle  States.  In  the  Southern  States,  it  is 
known  as  Spruce  Pine  and  Short-leaved  Pine. 

Long-leaved  Pine,  or  Southern  Pine,  (Pinus  Australis.)  This 
tree  has  but  little  sap-wood  :  and  the  resinous  matter  is  uniformly 
distributed  throughout  the  heart-wood,  which  presents  a  fine  com- 
pact grain,  having  more  hardness,  strength,  and  durability,  than 
any  other  species  of  the  pine,  owing  to  which  qualities  the  timber 
is  in  very  great  demand. 

The  tree  is  first  met  with  near  Norfolk,  Virginia,  and  from  this 
point  south,  it  is  abundantly  found. 

White  Pine,  or  Northern  Pine,  (Pinus  Strobus.)  This  tree 
takes  its  name  from  the  color  of  its  wood,  whicli  is  white,  soft 
light,  straight-grained,  and  durable.  It  is  inferior  in  strength  to 
the  species  just  described,  and  has,  moreover,  the  defect  of  swell- 
ing in  damp  weather.  Its  timber  is,  however,  in  great  demand 
as  a  good  building  material,  being  almost  the  only  kind  in  use  in 
the  Eastern  and  Northern  Stales,  for  the  frame-wark  and  joinery 
of  houses,  &c. 


IRON.  53 

The  finest  specimens  of  this  tree  grow  in  the  foref/s  of  Maine 
.  t  is  found  in  great  abundance  between  the  43d  ard  47th  paral 
lets,  N.  L. 

201.  Among  the  forest  trees  in  less  general  use  than  the  oak 
and  pine,  the  Locust,  the  Chesnut,  the  Red  Cedar,  ard  the  Larch, 
hold  the  first  place  for  hardness,  strength,  and  durability.  They 
are  chiefly  used  for  the  franc  3-work  of  vessels.  The  chesnut,  the 
locust,  and  the  cedar,  are  preferred  to  all  other  trees  for  posts. 

202.  The  Black,  or  Double  Spruce,  (Abies  Nigra,)  also  af- 
fords an  excellent  material,  its  timber  being  strong,  durable,  and 
light. 

203.  The  Juniper  or  White  Cedar,  and  the  Cypress,  are  verj 
celebrated  for  affording  a  material,  which  is  very  light,  and  of 
great  durability,  when  exposed  to  the  weather ;  owing  to  these 
qualities,  it  is  almost  exclusively  used  for  shingles  and  other  ex- 
terior coverings.  These  two  trees  are  found,  in  great  abundance, 
in  the  swamps  of  the  Southern  States. 

METALS. 

204.  The  metals  in  most  common  use  in  constructions  are 
Iron,  Copper,  Zinc,  Tin,  and  Lead. 

IRON. 

205.  This  metal  is  very  extensively  used  for  the  purposes  of 
the  engineer  and  architect,  both  in  the  state  of  Cast  Iron,  and 
Wrought  Iron. 

200.  Cast  Iron  is  one  of  the  most  valuable  building  materials, 
owing  to  its  great  strength,  hardness,  and  durability,  and  the  ease 
with  which  it  can  be  cast,  or  moulded,  into  the  best  forms,  for 
the  purposes  to  which  it  is  to  be  applied. 

207.  Cast  iron  is  divided  into  two  principal  varieties,  the  Gray 
cast  iron,  and  White  cast  iron.  There  exists  a  very  marked  dif- 
ference between  the  properties  of  these  two  varieties.  There 
are,  besides,  many  intermediate  varieties,  which  partake  more  or 
less  of  the  properties  of  these  two,  as  they  approach,  in  their  ex- 
ternal appearances,  nearer  to  the  one  or  the  other. 

20i.  Gray  cast  iron,  when  of  a  good  quality,  is  slightly  malle- 
able in  a  cold  state,  and  vvi''  J  .„«*  ieadily  to  the  action  of  the  file, 
when  the  hard  outside  coating  is  removed.  This  variety  is  also 
sometimes  termed  soft  gray  cast  iron ;  it  is  softer  and  toughei 
than  the  white  iron.  Wnen  broken,  the  surface  of  the  fracture 
presents  a  g.anular  structure  ;  the  color  is  gray ;  and  the  lustre 
is  what  is  termed  metallic,  resembling  small  brilliant  particles  of 
lead  strewed  over  the  surface. 

209.  White  cast  iron  is  very  hard  and  brittle  ;  when  recently 


56  BUILDING  MATERIALS. 

broken,  the  surface  of  the  fracture  presents  a  distinctly -marked 
crystalline  structure ;  the  color  is  white ;  and  lustre  vitieous,  or 
bearing  a  resemblance  to  the  reflected  light  from  an  aggregation 
of  small  crystals. 

210.  Mr.  Mallet,  in  a  very  able  Report  made  to  the  British 
Association  for  the  Advancement  of  Science,  remarking  on  the 
great  want  of  uniformity,  among  manufacturers  of  iron,  in  the 
terms  used  to  describe  its  different  varieties,  proposes  the  follow- 
ing nomenclature,  as  comprising  every  variety,  with  their  distinc- 
tive characters. 

Silvery.  Least  fusible  ;  thickens  rapidly  when  fluid  by  a 
spontaneous  puddling ;  crystals  vesicular,  often  crystalline  ;  in- 
capable of  being  cut  by  chisel  or  file ;  ultimate  cohesion  a  maxi- 
mum ;  elastic  range  a  minimum. 

Micaceous.  Very  soft ;  greasy  feel ;  peculiar  micaceous  ap- 
pearance generally  owing  to  excess  of  manganese  ;  soils  the  fin- 
gers strongly  ;  crystals  large  ;  runs  very  fluid  ;  contraction  large. 

Mottled.  Tough  and  hard  ;  filed  or  cut  with  difficulty  ;  crys- 
tals large  and  small  mixed  ;  sometimes  runs  thick  ;  contraction  in 
cooling  a  maximum. 

Bright  Gray.  Toughness  and  hardness  most  suitable  for 
working ;  ultimate  cohesion  and  elastic  range  generally  are  bal- 
anced most  advantageously  ;  crystals  uniform,  very  minute. 

Dull  Gray.  Less  tough  than  the  preceding  ;  other  characters 
alike  ;  contraction  in  cooling  a  minimum. 

Dark  Gray.  Most  fusible  ;  remains  long  fluid  ;  exudes  graphite 
in  cooling ;  soils  the  fingers  ;  crystals  large  and  lamellar  ;  ultimate 
cohesion  a  minimum,  and  elastic  range  a  maximum. 

211.  The  gray  iron  is  most  suitable  where  strength  is  required ; 
and  the  white,  where  hardness  is  the  principal  requisite. 

212.  The  color  and  lustre,  presented  by  the  surface  of  a  recent 
fracture,  are  the  best  indications  of  the  quality'  of  iron.  A  uni- 
form dark  gray  color,  and  high  metallic  lustre,  are  indications  of  the 
best  and  strongest.  With  the  same  color,  but  less  lustre,  the  iron 
will  be  found  to  be  softer  and  weaker,  and  to  crumble  readily. 
Iron  without  lustre,  of  a  dark  and  mottled  color,  is  the  softest  and 
weakest  of  the  gray  varieties. 

Iron  of  a  light  gray  color  and  high  metallic  lustre,  is  usually 
very  hard  and  tenacious.  As  the  color  approaches  to  white,  and 
the  metallic  lustre  changes  to  vitreous,  hardness  and  briltleness 
become  more  marked,  until  the  extremes  of  a  dull,  or  grayish  white 
color,  and  a  very  high  vitreous  lustre,  are  attained,  which  are  the 
indications  of  the  hardest  and  most  brittle  of  the  white  variety. 

213.  The  quality  of  cast  iron  may  also  be  tested,  by  striking  a 
smart  stroke  with  a  hammer  on  the  edge  of  a  casting.  If  the 
blow  produces  a  slight  indentation,  without  any  appearance  of 


IRON.  57 

fracture,  it  shows  that  the  iron  is  slightly  malleable,  and,  there- 
fore, of  a  good  quality  ;  if,  on  the  contrary,  the  edge  is  broken,  it 
indicates  brittleness  in  the  material,  and  a  consequent  want  of 
strength. 

214.  The  strength  of  cast  iron  varies  with  its  density  ;  and  this 
element  depends  upon  the  temperature  of  the  metal  when  drawn 
from  the  furnace ;  the  rate  of  cooling ;  the  head  of  metal  under 
which  the  casting  is  made  ;  and  the  bulk  of  the  casting. 

215.  The  density  of  iron  cast  in  vertical  moulds  increases,  ac- 
cording to  Mr.  Mallet's  experiments,  very  rapidly  from  the  top 
downward,  to  a  depth  of  about  four  feet  below  the  top  ;  from  this 
point  to  the  bottom,  the  rate  of  increase  is  very  nearly  uniform. 
All  other  circumstances  remaining  the  same,  the  density  decreases 
with  the  bulk  of  the  casting  ;  hence  large  are  proportionally 
weaker  than  small  castings. 

216.  From  all  of  these  causes,  by  which  the  strength  of  iron 
may  be  influenced,  it  is  very  difficult  to  judge  of  the  quality  of  a 
casting  by  its  external  characters  ;  in  general,  however,  if  the 
exterior  presents  a  uniform  appearance,  devoid  of  marked  ine- 
qualities of  surface,  it  will  be  an  indication  of  uniform  strength. 

217.  The  economy  in  the  manufacture  of  cast  iron,  arising 
from  the  use  of  the  hot  blast,  has  naturally  directed  attention  to 
the  comparative  merits  between  iron  produced  by  this  process 
and  that  from  the  cold  blast.  This  subject  has  been  ably  inves- 
tigated by  Messrs.  Fairbairn  and  Hodgkinson,  and  their  results 
published  in  the  Seventh  Report  of  the  British  Association. 

Mr.  Hodgkinson  remarks  on  this  subject,  in  reference  to  the 
results  of  his  experiments  :  "  It  is  rendered  exceedingly  probable 
that  the  introduction  of  a  heated  blast  into  the  manufacture  of 
cast  iron,  has  injured  the  softer  irons,  while  it  has  frequently 
mollified  and  improved  those  of  a  harder  nature  ;  and  considering 
the  small  deterioration  that"  some  "  irons  have  sustained,  and  the 
apparent  benefit  to  those  of"  others,  "  together  with  the  great  saving 
effected  by  the  heated  blast,  there  seems  good  reason  for  the  pro- 
cess becoming  as  general  as  it  has  done." 

218.  From  a  number  of  specific  gravities  given  in  these  Re- 
ports, the  mean  specific  gravity  of  cold  blast  iron  is  nearly  7.091, 
that  of  hot  blast  7.021. 

219.  Mr.  Fairbairn  concludes  his  Report  with  these  observa 
tions,  as  the  results  of  the  investigations  of  himself  and  Mr.  Hodg- 
kinson :  "  The  ultimatum  of  our  inquiries,  made  in  this  way, 
stands,  therefore,  in  the  ratio  of  strength,  1000  for  the  cold  blast, 
to  1024.8  for  the  hot  blast ;  leaving  the  small  fractional  difference 
of  24.8  in  favor  of  the  hot  blast." 

"  The  relative  powers  to  sustain  impact,  are  likewise  in  favo 
of  the  hot  blast,  being  in  the  ratio  of  1000  to  1226.3  " 

8 


58  BUILDING  MATERIALS 

220.  Wrought  Iron.  The  color,  lustre,  and  texture  of  a  recenl 
fracture,  present,  also,  the  most  certain  indications  of  the  quality 
of  wrought  iron.  The  fracture  submitted  to  examination,  should 
be  of  bars  at  least  one  inch  square  ;  or,  if  of  flat  bars,  they  should 
be  at  least  half  an  inch  thick  ;  otherwise,  the  texture  will  be  sc 
greatly  changed,  arising  from  the  greater  elongation  of  the  fibres, 
in  bars  of  smaller  dimensions,  as  to  present  none  of  those  dis- 
tinctive differences  observable  in  the  fracture  of  large  bars. 

221.  The  surface  of  a  recent  fracture  of  good  iron,  presents  a 
clear  gray  color,  and  high  metallic  lustre  ;  the  texture  is  granular, 
and  the  grains  have  an  elongated  shape,  and  are  pointed  and 
slightly  crooked  at  their  ends,  giving  the  idea  of  a  powerful  force 
having  been  employed  to  produce  the  fracture.  When  a  bar, 
presenting  these  appearances,  is  hammered,  or  drawn  out  into 
small  bars,  the  surface  of  fracture  of  these  bars  will  have  a  very 
marked  fibrous  appearance,  the  filaments  being  of  a  white  color 
and  very  elongated. 

222.  When  the  texture  is  either  laminated,  or  crystalline,  it  is 
an  indication  of  some  defect  in  the  metal,  arising  either  from  the 
mixture  of  foreign  ingredients,  or  else  from  some  neglect  in  the 
process  of  forging. 

223.  Burnt  iron  is  of  a  clear  gray  color,  with  a  slight  shade 
of  blue,  and  of  a  slaty  texture.     It  is  soft  and  brittle. 

224.  Cold  short  iron,  or  iron  that  cannot  be  hammered  when 
cold  without  breaking,  presents  nearly  the  same  appearance  as 
burnt  iron,  but  its  color  inclines  to  white.  It  is  very  hard  and 
brittle. 

225.  Hot  short  iron,  or  that  which  breaks  under  the  ham- 
mer when  heated,  is  of  a  dark  color  without  lustre.  This  de- 
fect is  usually  indicated  in  the  bar  by  numerous  cracks  on  the 
edges. 

226.  The  fibrous  texture,  which  is  developed  only  in  small 
bars  by  hammering,  is  an  inherent  quality  of  good  iron  ;  those 
varieties  which  are  not  susceptible  of  receiving  this  peculiar  tex- 
ture, are  of  an  inferior  quality,  and  should  never  be  used  for  pur- 
poses requiring  great  strength  :  the  filaments  of  bad  varieties  are 
short,  and  the  fracture  is  of  a  deep  color,  between  lead  gray  and 
dark  gray. 

227.  The  best  wrought  iron  presents  two  varieties  ;  the  Hard 
and  Soft.  The  hard  variety  is  very  strong  and  iuctiie.  It  pre- 
serves its  granular  texture  a  long  time  under  the  action  of  the 
hammer,  and  only  developes  the  fibrous  texture  when  beaten,  or 
drawn  out  into  small  rods :  its  filaments  then  present  a  silver 
white  appearance. 

228.  The  soft  variety  is  weaker  than  the  hard  ;  it  yields  easily 
tc  the  hammer;  and  it  commences  to  ex  libit,  under  ils  action 


IRON.  59 

the  fioioua  texture  in  tolerably  large  bars.     The  color  of  the 
fibres  is  between  a  silver  white  and  lead  gray. 

229.  Iron  may  be  naturally  of  a  good  quality,  and  stil ,  from 
being  badly  refined,  not  present  the  appearances  which  are  re- 
garded as  sure  indications  of  its  excellence.  Among  the  defects 
arising  from  this  cause  are  blisters,  flaws,  and  cinder-holes. 
Generally,  however,  if  the  surface  of  fracture  presents  a  texture 
partly  crystalline  and  partly  fibrous,  or  a  fine  granular  texture,  in 
which  some  of  the  grains  seem  pointed  and  crooked  at  the  points, 
together  with  a  light  gray  color  without  lustre,  it  will  indicate 
natural  good  qualities,  which  require  only  careful  refining  to  be 
fully  developed. 

230.  The  strength  of  wrought  iron  is  very  variable,  as  it  de- 
pends not  only  on  the  natural  qualities  of  the  metal,  but  also  upon 
the  care  bestowed  in  forging,  and  the  greater  or  less  compres- 
sion of  its  fibres,  when  drawn  or  hammered  into  bars  of  different 
sizes. 

231.  In  the  Report  made  by  the  sub-committee,  Messrs.  John 
son  and  Reeves,  on  the  strength  of  Boiler  Iron,  {Journal  of  Frank- 
lin Institute,  vol.  20,  New  Series,)  it  is  stated  that  the  following 
order  of  superiority  obtains  among  the  different  kinds  of  pig 
metal,  with  respect  to  the  malleable  iron  which  they  furnish  : — 
1  Lively  gray ;  2  White ;  3  Mottled  gray ;  4  Dead  gray ; 
5  Mixed  metals. 

The  Report  states,  "  So  far  as  these  experiments  may  be  con 
sidered  decisive  of  the  question,  they  favor  the  lighter  complexion 
of  the  cast  metal,  in  preference  to  the  darker  and  mottled  varie 
ties  ;  and  they  place  the  mixture  of  different  sorts  among  the 
worst  modifications  of  the  material  to  be  used,  where  the  object 
is  mere  tenacity." 

232.  These  experiments  also  show  that  piling  iron  of  different 
degrees  of  fineness  in  the  same  plate  is  injurious  to  its  quality, 
owing  to  the  consequent  inequality  of  the  welding. 

233.  From  these  experiments,  the  mean  specific  gravity  of 
boiler  iron  is  7.7344,  and  of  bar  iron  7.7254. 

234.  Durability  of  Iron.  The  durability  of  iron,  under  the 
different  circumstances  of  exposure  to  which  it  may  be  submitted, 
depends  on  the  manner  in  which  the  casting  may  have  been  made; 
the  bulk  of  the  piece  employed  ;  the  more  or  less  homogeneous- 
ness  of  the  mass  ;  its  density  and  hardness. 

235.  Among  the  most  recent  and  able  researches  upon  the  ac 
lion  of  the  ordinary  corrosive  agents  on  iron,  and  the  preservative 
means  to  be  employed  against  them,  those  of  Mr.  Mallet,  given  in 
the  Report  already  mentioned,  hold  the  first  rank.  A  brief  re- 
capitulation of  the  most  prominent  conclusions  at  which  he  has 
arrived,  is  all  that  can  be  attempted  in  this  place. 


60  BUILDING  MATERIALS. 

236.  When  iron  is  only  partly  immersed  in  water,  or  wholly 
immersed  in  water  composed  of  strata  of  different  densities,  like 
that  of  tidal  rivers,  a  voltaic  pile  of  one  solid  and  twi  fluid  bodies 
is  formed,  which  causes  a  more  rapid  corrosion  than  when  the 
liquid  is  of  uniform  density. 

237.  The  corrosive  action  of  the  foul  sea  water  of  docks  and 
harbors  is  far  more  powerful  than  that  of  clear  sea,  or  fresh  water, 
owing  to  the  action  of  the  hydro-sulphuric  acid  which,  being  dis- 
engaged from  the  mud,  impregnates  the  water,  and  acts  on  the 
iron. 

238.  In  clear  fresh  river  water,  the  corrosive  action  is  less  than 
under  any  other  circumstances  of  immersion ;  owing  to  the  ab- 
sence of  corrosive  agents,  and  the  firm  adherence  of  the  oxide 
formed,  which  presents  a  hard  coat  that  is  not  washed  off  as  in 
sea  water. 

239.  In  clear  sea  water,  the  rate  of  corrosion  of  iron  bars,  one 
inch  thick,  is  from  3  to  4  tenths  of  an  inch  for  cast  iron  in  a  cen- 
tury, and  about  6  tenths  of  an  inch  for  wrought  iron. 

240.  Wrought  iron  corrodes  more  rapidly  in  hot  sea  water  than 
under  any  other  circumstances  of  immersion. 

241.  The  same  iron  when  chill  cast  corrodes  more  rapidly  than 
when  cast  in  green  sand ;  this  arises  from  the  chilled  surface 
being  less  uniform,  and  therefore  forming  voltaic  couples  of  iron 
of  different  densities,  by  which  the  rapidity  of  corrosion  is  in- 
creased. 

242.  Castings  made  in  dry  sand  and  loam  are  more  durable 
under  water  than  those  made  in  green  sand. 

243.  Thin  bars  of  iron  corrode  more  rapidly  than  those  of  more 
bulk.  This  difference  in  the  rate  of  corrosion  is  more  striking  in 
the  soft,  or  graphitic  specimens  of  cast  iron,  than  in  the  hard  and 
silvery.  It  is  caused  by  the  more  rapid  rate  of  cooling  in  thin 
than  in  thick  bars,  by  which  the  density  of  the  surface  of  the  for- 
mer becomes  less  uniform.  These  causes  of  destructibility  may, 
in  some  degree,  be  obviated  in  castings  composed  of  ribbed 
pieces,  by  making  the  ribs  of  equal  thickness  with  the  main 
pieces,  and  causing  them  to  be  cooled  in  the  sand,  before  strip- 
ping the  moulds. 

244.  The  hard  crust  of  cast  iron  promotes  its  durability  ;  when 
this  is  removed  to  the  depth  of  one  fourth  of  an  inch,  the  iron  cor- 
rodes more  rapidly  in  both  air  and  water. 

245.  Corrosion  takes  place  the  less  rapidly  in  any  variety  of 
iron,  in  proportion  as  it  is  more  homogeneous,  denser,  harder,  and 
•.loser  grained,  and  the  less  graphitic. 

246.  The  more  ordinary  means  used  to  protect  iron  against 
ihe  action  of  corrosive  agents,  cons.st  of  paints  and  varnishes, 
These,  in  der  the  usual  circumstances  of  atmospheric  exposure 


IRON.  61 

are  of  but  slight  efficacy,  and  require  to  be  frequently  renewed 
In  water,  they  are  all  rapidly  destroyed,  with  the  exception  of 
boiled  coal-tar,  which,  when  laid  on  hot  iron,  leaves  a  bright  anq 
solid  varnish  of  considerable  durability  and  protective  power. 

247.  The  rapidly  increasing  purposes  to  which  iron  has  been 
applied,  within  the  last  few  years,  has  led  to  reseaiches  upon  the 
agency  of  electro-chemical  action,  as  a  means  of  protecting  it  from 
corrosion,  both  in  air  and  water.  Among  the  processes  resorted 
to  for  this  purpose,  that  of  zincking,  or  as  it  is  more  commonly 
known,  galvanizing  iron  has  been  most  generally  introduced. 
The  experiments  of  Mr.  Mallet,  on  this  process,  are  decidedly 
against  zinc  as  a  permanent  electro-chemical  protector.  Mr. 
Mallet  states,  as  the  result  of  his  observations,  that  zinc  applied  in 
fusion,  in  the  ordinary  manner,  over  the  whole  surface  of  iron, 
will  not  preserve  it  longer  than  about  two  years ;  and  that,  so 
soon  as  oxidation  commences  at  any  point  of  the  iron,  the  protec- 
tive power  of  the  zinc  becomes  considerably  diminished,  or  even 
entirely  null.  Mr.  Mallet  concludes,  "  On  the  whole,  it  may  be 
affirmed  that,  under  all  circumstances,  zinc  has  not  yet  been  so 
applied  to  iron,  as  to  rank  as  an  electro-chemical  protector  to- 
wards it  in  the  strict  sense  ;  hitherto  it  has  not  become  a  preven- 
tive, but  merely  a  more  or  less  effective  palliative  to  destruction." 

248.  In  extending  his  researches  on  this  subject,  with  alloys 
of  copper  and  zinc,  and  copper  and  tin,  Mr.  Mallet  found  that  the 
alloys  of  the  last  metals  accelerate  the  corrosion  of  iron,  when 
voltaically  associated  with  it  in  sea  water  ;  and  that  an  alloy  of 
the  two  first,  represented  by  23 Zn  4-  8Cu,  in  contact  with  iron, 
protects  it  as  fully  as  zinc  alone,  and  suffers  but  little  loss  from 
the  electro-chemical  action  ;  thus  presenting  a  protective  en- 
ergy more  permanent  and  invariable  than  that  of  zinc,  and  giving 
a  nearer  approximation  to  the  solution  of  the  problem,  "  to  obtain 
a  mode  of  electro-chemical  protection  such,  that  while  the  iron 
shall  be  preserved  the  protector  shall  not  be  acted  on,  and  whose 
protection  shall  be  invariable." 

249.  In  the  course  of  his  experiments,  Mr.  Mallet  ascertained 
that  the  softest  gray  cast  iron  bears  such  a  voltaic  relation  to  hard 
bright  cast  iron,  when  immersed  in  sea  water  and  voltaically  as- 
sociated with  it,  that  although  oxidation  will  not  be  prevented  on 
either,  it  will  still  be  greatly  retarded  on  the  hard,  at  the  expense 
of  the  soft  iron. 

250.  In  concluding  the  details  of  his  important  researches  on 
this  subject,  Mr.  Mallet  makes  the  following  judicious  remarks  : 
"  The  engineer  of  observant  habit  will  soon  have  perceived,  thai 
in  exposed  works  in  iron,  equality  of  section  or  scantling,  in  all 
parts  sustaining  equal  strain,  is  far  from  ensuring  equal  passive 
power  of  permanent  resistance,  unless,  in  addition  to  a  genera] 


62  BOILDING  MATERIALS. 

allowance  for  loss  of  substance  by  corrosion,  this  latter  element 
be  so  provided  for,  that  it  shall  be  equally  balanced  over  the  whole 
structure ;  or,  if  not,  shall  be  compelled  to  confine  itself  to  por- 
tions of  the  general  structure,  which  may  lose  substance  without 
injuring  its  stability." 

"  The  principles  we  have  already  est.  jlished  sufficiently  g  lide 
us  in  the  modes  of  effecting  this  ;  regard  must  not  only  be  had  to 
the  contact  of  dissimilar  metals,  or  of  the  same  in  dissimilar  fluids, 
but  to  the  scantling  of  the  casting  and  of  its  parts,  and  to  the  con- 
tact of  cast  iron  with  wrought  iron  or  steel,  or  of  one  sort  of  cast 
iron  with  another.  Thus,  in  a  suspension  bridge,  if  the  links  of 
the  chains  be  hammered,  and  the  pins  rolled,  the  latter,  where 
equally  exposed,  will  be  eaten  away  long  before  the  former.  In 
marine  steam-boilers,  the  rivets  are  hardened  by  hammering  until 
cold  ;  the  plates,  therefore,  are  corroded  through  round  the  rivets 
before  these  have  suffered  sensibly ;  and  in  the  air-pumps  and 
condensers  of  engines  working  with  sea  water,  or  in  pit  work,  and 
pumps  lifting  mineralized  or  'bad' water  from  mines,  the  cast 
iron  perishes  first  round  the  holes  through  which  wrought-iron 
bolts,  &c,  are  inserted.  And  abundant  other  instances  might  be 
given,  showing  that  the  effects  here  spoken  of  are  in  practical 
operation  to  an  extent  that  should  press  the  means  of  counteract- 
ing them  on  the  attention  of  the  engineer." 

251.  Since  Mr.  Mallet's  Report  to  the  British  Association,  he 
has  invented  two  processes  for  the  protection  of  iron  from  the  ac- 
tion of  the  atmosphere  and  of  water ;  the  one  by  means  of  a  coat- 

ng  formed  of  a  triple  alloy  of  zinc,  mercury,  and  sodium,  or  po- 
tassium ;  the  other  by  an  amalgam  of  palladium  and  mercury. 

252.  The  first  process  consists  of  forming  an  alloy  of  the  metals 
used,  in  the  following  manner.  To  1292  parts  of  zinc  by  weight, 
in  a  state  of  fusion,  202  parts  of  mercury  are  added,  and  the 
metals  are  well  mixed,  by  stirring  writh  a  rod  of  dry  wrood,  or  one 
of  iron  coated  with  clay ;  sodium,  or  potassium  is  next  added,  in 
small  quantities  at  a  time,  in  the  proportion  of  one  pound  to  every 
ton  by  weight  of  the  other  two  metals.  The  iron  to  be  coated 
with  this  alloy  is  first  cleared  of  all  adhering  oxide,  bv  immersing 
it  in  a  warm  dilute  solution  of  sulphuric,  or  of  hydro-chloric  acid, 
vashing  it  in  clear  cold  water,  and  detaching  all  scales,  by  striking 
it  with  a  hammer ;  it  is  then  scoured  clean  by  the  hand  with  sand, 
or  emery,  under  a  small  stream  of  water,  until  a  bright  metallic 
lustre  is  obtained  ;  while  still  wet,  it  is  'mmersed  in  a  bath  formed 
of  equal  parts  of  the  cold  saturated  solutions  of  chloride  of  zinc 
and  sal-ammoniac,  to  which  as  much  more  solid  sal-ammoniac  is 
added  as  the  solution  will  take  up.  The  iron  is  allowed  to  re- 
main in  this  bath  until  it  is  covered  by  minute  bubbles  of  gas  ;  it 
is  then  taken  out,  allowed  to  drain  a  few  seconds,  and  plunged 


COPPER ZINC.  63 

into  the  fused  alloy,  from  which  it  is  withdrawn  so  soon  as  it  nas 
acquired  the  same  temperature.  When  taken  from  the  metallic 
bath,  the  iron  should  be  plunged  in  cold  water  and  well  washed. 

253.  Care  must  be  taken  that  the  iron  be  not  kept  too  long  in 
the  metallic  bath,  otherwise  it  may  be  fused,  owing  to  the  great 
affinity  of  the  alloy  for  iron.  At  the  proper  fusing  temperature 
of  the  alloy,  about  680°  Fahr.,  it  will  dissolve  plates  of  iron  one 
eighth  of  an  inch  thick  in  a  few  seconds  ;  on  this  account,  when- 
ever small  articles  of  iron  have  to  be  protected,  the  affinity  of  the 
alloy  for  iron  should  be  satisfied,  by  fusing  some  iron  in  it  before 
immersing  that  to  be  coated. 

254.  The  other  process,  which  has  been  termed palladiumizing, 
consists  in  coating  the  iron,  prepared  as  in  the  first  process  for 
the  reception  of  the  metallic  coat,  with  an  amalgam  of  palladium 
and  mercury. 

COPPER. 

255.  The  most  ordinary  and  useful  application  of  this  metal  in 
constructions,  is  that  of  sheet  copper,  which  is  used  for  roof  cov- 
erings, and  like  purposes.  Its  durability  under  the  ordinary 
changes  of  atmosphere  is  very  great.  Sheet  copper,  when  quite 
thin,  is  apt  to  be  defective,  from  cracks  arising  from  the  process 
of  drawing  it  out.  These  may  be  remedied,  when  sheet  copper 
is  to  be  used  for  a  water-tight  sheathing,  by  tinning  the  sheets  on 
one  side.  Sheets  prepared  in  this  way  have  been  found  to  be  very 
durable. 

The  alloys  of  copper  and  zinc,  known  under  the  name  of  brass, 
and  those  of  copper  and  tin,  known  as  bronze,  gun-metal,  and 
bell-metal,  are,  in  some  cases,  substituted  for  iron,  owing  to  their 
superior  hardness  to  copper,  and  being  less  readily  oxidized  than 
iron. 

ZINC. 

256.  This  metal  is  used  mostly  in  the  form  of  sheets  ;  and  for 
water-tight  sheathings  it  has  nearly  displaced  every  other  kind  of 
sheet  metal.  The  pure  metallic  surface  of  zinc  soon  becomes 
covered  with  a  very  thin,  hard,  transparent  oxide,  which  is  un 
changeable  both  in  air  and  water,  and  preserves  the  metal  beneath 
it  from  farther  oxidation.  It  is  this  property  of  the  oxide  of  zinc, 
which  renders  this  metal  so  valuable  for  sheathing  purposes  ;  but 
its  durability  is  dependent  upon  its  not  being  brought  into  contact 
with  iron  in  the  presence  of  moisture,  as  the  galvanic  action  which 
would  then  ensue,  would  soon  destroy  the  zinc.  On  the  same 
account  zinc  should  be  perfectly  free  from  the  presence  of  iron, 
as  a  very  small  quantity  of  the  oxide  of  this  last  metal  when  con 
lained  in  zinc,  is  found  to  occasion  its  rapid  destruction. 


64  BUILDING   MATERIALS. 

257.  Besides  the  alloys  of  zinc  already  mentioned,  this  meta. 
alloyed  with  copper  forms  one  of  the  most  useful  solders  ;  and 
its  alloy  with  lead  has  been  proposed  as  a  cramping  metal  for 
uniling  the  parts  of  iron  work  together,  or  iron  work  to  other  ma- 
terials, in  the  place  of  lead,  which  is  usually  employed  for  this 
purpose,  but  which  accelerates  the  destruction  of  iron  in  contact 
with  it. 

TIN. 

258.  The  most  useful  application  of  tin  is  as  a  coating  for 
sheet  iron,  or  sheet  copper  :  the  alloy  which  it  forms,  in  this  way, 
upon  the  surfaces  of  the  metals  in  question,  preserves  them  for 
some  time  from  oxidation.  Alloyed  with  lead  it  forms  one  of  the 
most  useful  solders. 

LEAD. 

259.  Lead  in  sheets  forms  a  very  good  and  durable  roof  cover- 
ing, but  it  is  inferior  to  both  copper  and  zinc  in  tenacity  and 
durability ;  and  is  very  apt  to  tear  asunder  on  inclined  surfaces, 
particularly  if  covered  with  other  materials,  as  in  the  case  of  the 
capping  of  water-tight  arches. 

PAINTS  AND  VARNISHES. 

260.  Paints  are  mixtures  of  certain  fixed  and  volatile  oils, 
chiefly  those  of  linseed  and  turpentine,  with  several  of  the  metal 
lie  salts  and  oxides,  and  other  substances  which  are  used  either 
as  pigments,  or  to  give  what  is  termed  a  body  to  the  paint,  and 
also  to  improve  its  drying  properties. 

261.  Paints  are  mainly  used  as  protective  agents  to  secure 
wood  and  metals  from  the  destructive  action  of  air  and  water. 
This  they  but  imperfectly  effect,  owing  to  the  unstable  nature  of 
the  oils  that  enter  into  their  composition,  which  are  not  only  de- 
stroyed by  the  very  agents  against  which  they  are  used  as  pro- 
tectors, but  by  the  chemical  changes  which  result  from  the  action 
of  the  elements  of  the  oil  upon  the  metallic  salts  and  oxides. 

262.  Paints  are  more  durable  in  air  than  in  water.  In  the  lat- 
ter element,  whether  fresh  or  salt,  particularly  if  foul,  paints  are 
soon  destroyed  by  the  chemical  changes  which  take  place,  both 
from  the  action  of  the  water  upon  the  oils,  and  that  of  the  hydro- 
sulphuric  acid  contained  in  foul  water  upon  the  metallic  salts  and 
oxiaes. 

'263.  However  carefully  made  or  applied,  paints  soon  become 
permeable  to  water,  owing  to  the  very  minute  pores  which  arise 
from  the  chemical  changes  in  their  constituents.  These  changes 
will  have  but  little  influence  upon  the  preservative  acticn  of  paints 
upon  wood  exposed  lo  the  effects  of  the  atmosphere,  provided  the 
wood  l»e  well  seasoned  before  the  paint  is  applied,  and  that  the 


PAINTS  AND  VARNISHES. 


65 


latter  be  renewed  at  suitable  intervals  of  time.  On  metals  these 
changes  have  a  very  important  bearing.  The  permeability  of  the 
paint  to  moisture  causes  the  surface  of  the  metal  under  it  to  rust, 
and  this  cause  of  destruction  is,  in  most  cases,  promoted  by  the 
chemical  changes  which  the  paint  undergoes. 

264.  Varnishes  are  solutions  of  various  resinous  substances 
in  solvents  which  possess  the  property  of  drying  rapidly.  They 
are  used  for  the  same  purposes  as  paints,  and  have  generally  the 
same  defects. 

265.  The  following  are  some  of  the  more  usual  compositions 
of  paints  and  varnishes. 


White  Paint,  {for  exposed  wood.) 

White  lead,  ground  in  oil 

Boiled  oil        .... 

Raw  oil  . 

Spirits  turpentine     .... 


80 
9 
9 

4 


The  white  lead  to  be  ground  in  the  oil,  and  the  spirits  of  tui 
pentine  added. 

Black  Paint. 


Lamp-black     . 
Litharge 
Japan  varnish 
Linseed  oil,  boiled 
Spirits  turpentine 


Lead  Color. 


White  lead,  ground  in  oil 
Lamp-black     . 
Boiled  linseed  oil     . 
Litharge 
Japan  varnish 


28 
1 
1 

73 
1 


75 
1 

23 
0.5 
0.5 


Spirits  turpentine 2.5 

Gray,  or  Stone  Color,  (for  buildings.) 


White  lead  ground  in  oil 

Boiled  oil 

Raw  oil  . 

Spirits  of  turpentine 

Turkey  umber 

Lamp-black     . 


78 
9.5 
9.5 
3 

0.5 
0.25 


Lackers  for  Cast  Iron. 


1  —Black  lead,  pulverized 
Red  lead 
Litharge 
Lamp-black     . 
Linseed  oil 


12 

12 

6 

5 

68 


66  BUILDING  MATERIALS. 


2  — Anti-corrosion 

Grant's  black,  ground  in  oil 
Red  lead,  as  a  dryer 
Linseed  oil      . 
Spirits  turpentine     . 


40  lbs. 

4  " 

3  " 

4  gall. 
1  pint. 


Copal  Varnish. 


Gum  copal,  (in  dean  lumps)    .         .         .     26.5 

Boiled  linseed  oil 42.5 

Spirits  turpentine    .         .         .         .         .31 

Japan  Varnish. 

Litharge          ......  4 

Boiled  oU 87 

Spirits  turpentine 2 

Red  lead 6 

Umber 1 

Gum  shellac 8 

Sugar  of  lead 2 

White  vitriol 1 

The  proportions  of  the  above  compositions  are  giver*  #•  i00 
parts,  by  weight,  with  the  exception  of  lacker  2. 

The  beautiful  black  polish  on  the  Berlin  castings  for  ornt  mental 
purposes,  is  said  to  be  produced  by  laying  the  following  compo- 
sition on  the  hot  iron,  and  then  baking  it. 

Bitumen  of  India     .....     0.5 

Resin 0.5 

Drying  oil 1.0 

Copal,  or  amber  varnish  .         .         .         .1.0 

Enough  oil  of  turpentine  is  to  be  added  to  this  mixture  to  make 
it  spread. 

266.  From  experiments  made  by  Mr.  Mallet,  on  the  preserva- 
tive properties  of  paints  and  varnishes  for  iron  immersed  in  water, 
it  appears  that  caoutchouc  varnish  is  the  best  for  iron  in  hot 
water,  and  asphaltum  varnish  under  all  other  circumstances  ;  but 
that  boiled  coal-tar,  laid  on  hot  iron,  forms  a  superior  coating  to 
either  of  the  foregoing. 

267.  Mr.  Mallet  recommends  the  following  compositions  for  a 
paint,  termed  by  him  zoofagous  paint,  and  a  varnish  to  be  used 
to  preserve  zincked  iron  both  from  corrosion  and  from  fouling  in 
sea  water. 

Varnish  for  zincked  Iron. 

To  50  lbs.  of  foreign  asphaltum,  melted  and  boiled  in  an  iron 
vessel  for  three  or  four  hours,  add  16  lbs.  of  red  lead  and  litharge 
ground  to  a  fine  powder,  in  equal  proportions,  with  1 0  gals,  of 


STRENGTH  OF  MATERIALS  67 

drying  linseed  oil,  and  bring  the  whole  .to  a  nearly  boiling  tem- 
perature. Melt,  in  a  second  vessel,  8  lbs.  of  gum-anime ;  to 
which  add  2  gals,  of  drying  linseed  oil  at  a  boiling  heat,  with  12 
lbs.  of  caoutchouc  partially  dissolved  in  coal-tar  naphtha.  Pour 
the  contents  of»the  second  vessel  into  the  first,  and  boil  the  whole 
gently,  until  the  varnish,  when  taken  up  between  two  spatulas,  is 
found  to  be  tough  and  ropy.  This  composition,  when  quite  cold, 
is  to  be  thinned  down  for  use  with  from  30  to  35  gals,  of  spirit! 
of  turpentine,  or  of  coal  naphtha. 

268.  It  is  recommended  that  the  iron  should  be  heated  before 
receiving  this  varnish,  and  that  it  should  be  applied  with  a  spatula, 
or  a  flexible  slip  of  horn,  instead  of  the  ordinary  brush. 

When  dry  and  hard,  it  is  stated  that  this  varnish  is  not  acted 
upon  by  any  moderately  diluted  acid,  or  alkali ;  and,  by  long  im- 
mersion in  water,  it  does  not  form  a  partially  soluble  hydrate,  as 
is  the  case  with  purely  resinous  varnishes  and  oil  paints.  It  can 
with  difficulty  be  removed  by  a  sharp-pointed  tool ;  and  is  so 
elastic,  that  a  plate  of  iron  covered  with  it  may  be  bent  several 
times  before  it  will  become  detached. 

Zoofagous  Paint. 

269.  To  100  lbs.  of  a  mixture  of  drying  linseed  oil,  red  lead, 
sulphate  of  barytes,  and  a  little  spirits  of  turpentine,  add  20  lbs. 
of  the  oxychloride  of  copper,  and  3  lbs.  of  yellow  soap  and  com- 
mon rosin,  in  equal  proportions,  with  a  little  water. 

When  zincked  iron  is  exposed  to  the  atmosphere  alone,  the  var- 
nish is  a  sufficient  protection  for  it ;  but  when  it  is  immersed  in 
sea  water,  and  it  is  desirable,  as  in  iron  ships,  to  prevent  it  from 
fouling,  by  marine  plants  and  animals  attaching  themselves  to  it, 
the  paint  should  be  used,  on  account  of  its  poisonous  qualities. 
The  paint  is  applied  over  the  varnish,  and  is  allowed  to  harden 
three  or  four  days  before  immersion. 


RESULTS  OF  EXPERIMENTAL  RESEARCHES  ON  THE 
STRENGTH  OF  MATERIALS. 

270.  Whatever  may  be  the  physical  structure  of  materials, 
whether  fibrous  or  granular,  experiment  has  shown  that  they  all 
possess  certain  general  properties,  among  the  most  important  of 
which  to  the  engineer  are  those  of  contraction,  elongation,  de~ 
flection,  torsion,  and  lateral  adhesion,  and  the  resistances  which 
these  offer  to  the  forces  by  which  they  are  called  into  action.* 
*  Ste  Note  !>.,  Appendix. 


03  BUILDING  MATERIALS. 

271  All  solid  bodies^  when  submitted  to  strains  by  which  an} 
ol  these  properties  are  developed,  have,  within  certain  limits, 
termed  the  limits  of  elasticity,  the  property  of  wholly  or  partially 
resuming  their  original  state,  when  the  strain  is  taken  off.  This 
property  is  usually  denominated  the  elastic  fores,  and  has  for  its 
measure,  in  the  case  of  contraction,  or  elongation,  the  ratio  be- 
tween the  force  which  causes  the  one  or  the  other  of  these  states 
and  the  fraction  which  measures  the  degree  of  contraction,  or 
elongation. 

272.  To  what  extent  bodies  possess  the  property  of  total  re- 
covery of  form,  when  relieved  from  a  strain,  is  still  a  matter  of 
doubt.  It  has  been  generally  assumed,  that  the  elasticity  of  a 
material  does  not  undergo  permanent  injury  by  any  strain  less 
than  about  one  third  of  that  which  would  entirely  destroy  its  force 
of  cohesion,  thereby  causing  rupture.  But  from  the  most  recent 
experiments  on  this  point  made  by  Mr.  Hodgkinson  on  cast  iron, 
it  appears  that  the  restoring  power  of  this  material  is  destroyed  by 
very  slight  strains ;  and  it  is  rendered  probable  that  this  and  most 
other  materials  receive  a  permanent  change  of  form,  or  set,  under 
any  strain,  however  small. 

273.  The  extension,  or  contraction  of  a  solid,  may  be  effected 
either  by  a  force  acting  in  the  direction  in  which  the  contraction, 
or  elongation  takes  place,  or  by  one  acting  transversely,  so  as  to 
bend  the  body.  Experiments  have  been  made  to  ascertain,  di- 
rectly, the  proportion  between  the  amount  of  contraction,  or  elon- 
gation, and  the  forces  by  which  they  are  produced.  From  these 
experiments,  it  results,  that  the  contractions,  or  elongations  are, 
within  certain  limits,  proportional  to  the  forces,  but  that  an  equal 
amount  of  contraction,  or  elongation,  is  not  produced  by  the  same 
amount  of  force.  From  the  experiments  of  Mr.  Hodgkinson  and 
M.  Duleau,  it  appears,  that  in  cast  and  malleable  iron  the  con- 
traction, or  elongation,  caused  by  the  same  amount  of  pressure, 
or  tension,  is  nearly  equal ;  while  in  timber,  according  to  Mr. 
Hodgkinson,  the  amount  of  contraction  is  about  four  fifths  of  the 
elongation  for  the  same  force. 

274.  When  a  solid  of  any  of  the  materials  used  in  construc- 
tions is  acted  upon  by  a  force  so  as  to  produce  deflection,  experi- 
ment has  shown  that  the  fibres  towards  the  concave  side  of  the 
bent  solid  are  contracted,  while  those  towards  the  convex  side 
are  elongated ;  and  that,  between  the  fibres  which  are  contracted 
and  those  which  are  elongated,  others  are  found  which  have  not 
undergone  any  change  of  length.  The  part  of  the  solid  occupied 
by  these  last  fibres  has  received  the  name  of  the  neutral  line,  or 
neutral  axis. 

275.  The  hypothesis  usually  adopted,  with  respect  to  the  cir- 
cumstances attending  tliis  kind  of  strain,  is  that  the  contraction! 


STRENGTH  OF  MATERIALS.  69 

and  elongations  of  the  fibres  on  each  side  of  the  neutral  axis  are 
proportional  to  their  distances  from  this  line  ;  and  that,  for  slight 
deflections,  the  neutral  axis  passes  through  the  centre  of  gravity 
of  the  sectional  area.  From  experiments,  however,  by  Mr.  Hodg- 
kinson  and  Mr.  Barlow,  it  appears  that  the  neutral  axis,  in  forged 
iron  and  cast  iron,  lies  nearer  to  the  concave  than  to  the  convex 
surface  of  the  bent,  solid,  and,  probably,  shifts  its  position  when 
the  degree  of  deflection  is  so  great  as  to  cause  rupture.  In  tim- 
ber, according  to  Mr.  Barlow,  the  neutral  axis  lies  nearest  to  the 
convex  surface ;  and,  from  his  experiments  on  solids  of  forged 
iron  and  timber  with  a  rectangular  sectional  figure,  he  places  the 
neutral  axis  at  about  three  eighths  of  the  depth  of  the  section  from 
the  convex  side  in  timber,  and  between  one  third  and  one  fifth  of 
the  depth  of  the  section  from  the  concave  side  in  forged  iron. 

276.  When  the  strain  to  which  a  solid  is  subjected  is  suffi- 
ciently great  to  destroy  the  cohesion  between  its  particles,  and 
cause  rupture,  experiment  has  shown  that  the  force  producing 
this  effect,  whether  it  act  by  tension,  so  as  to  draw  the  fibres 
asunder,  or  by  compression,  to  crush  them,  is  proportional  to  the 
sectional  area  of  the  solid.  The  measure,  therefore,  of  the  re- 
sistance offered  by  a  solid  to  rupture,  in  either  of  these  cases,  is 
that  force  which  will  rupture  a  sectional  area  of  the  solid  repre 
sented  by  unity. 

277.  From  experiments  made  to  ascertain  the  circumstances 
of  rupture  by  a  tensile  force,  it  appears  that  the  solid  torn  apart 
exhibits  a  surface  of  fracture  more  or  less  even,  according  to  the 
nature  of  the  material. 

278.  Most  of  the  experiments  on  the  resistance  to  rupture 
by  compression,  have  been  made  on  small  cubical  blocks,  and 
have  given  a  measure  of  this  resistance  greater  than  can  be  de- 
pended upon  in  practical  applications,  when  the  height  of  the 
solid  exceeds  three  times  the  radius  of  its  base.  This  point  has 
been  very  fully  elucidated  in  the  experiments  of  Mr.  Hodg- 
kinson  upon  the  rupture  by  compression  of  solids  with  circular 
and  rectangular  bases.  These  experiments  go  to  prove,  that  the 
circumstances  of  rupture,  and  the  resistance  offered  by  the  solid, 
vary  in  a  constant  manner  with  its  height,  the  base  remaining  the 
same.  In  columns  of  cast  iron,  with  circular  sectional  area-,  it 
was  found  that  the  resistance  remained  constant  for  a  height  less 
than  three  times  the  radius  of  the  base  ;  that,  from  this  height  to 
one  equal  to  six  times  the  radius  of  the  base,  the  resistance  still 
remained  constant,  but  was  less  than  in  the  former  case ;  anc 
that,  for  any  height  greater  than  six  times  the  radius  of  the  base, 
the  resistance  decreased  with  the  height.  In  the  two  first  cases 
the  solids  were  found  to  yield  either  by  the  upper  portion  sliding 
off  upon  the  lower,  in  the  direction  of  a  plane  making  a  constan 


70 


BUILDING  MATERIALS. 


angle  with  the  axis  of  the  solid ;  or  else  by  separating  into  coni 
cal,  or  wedge-shaped  blocks,  having  the  upper  and  lower  surfaces 
of  the  solid  as  their  bases,  the  angle  at  the  apex  being  double  that 
made  by  the  plane  and  axis  of  the  solid.  With  regard  to  the  re- 
sistances, it  was  fcund  that  they  varied  in  the  ratio  of  the  area  of 
the  bases  of  the  solids.  Where  the  height  of  the  solid  was  greater 
than  six  times  the  radius  of  the  base,  rupture  generally  took  place 
by  bending. 

279.  From  experiments  by  Mr.  Hodgkinson,  on  wood  and 
other  substances,  it  would  appear  that  like  circumstances  accom- 
pany the  rupture  of  all  materials  by  compression  ;  that  is,  within 
certain  limits,  they  all  yield  by  an  oblique  surface  of  fracture,  the 
angle  of  which  with  the  axis  of  the  solid  is  constant  for  the  same 
material ;  and  that  the  resistances  offered  within  these  limits  are 
proportional  to  the  areas  of  the  bases. 

260.  Among  the  most  interesting  deductions  drawn  by  Mr. 
Hodgkinson,  from  the  wide  range  of  his  experiments  upon  the 
strength  of  materials,  is  the  one  which  points  to  the  existence 
of  a  constant  relation  between  the  resistances  offered  by  materials 
of  the  same  kind  to  rupture  from  compression,  tension,  and  a 
transverse  strain.  The  following  Table  gives  these  relations, 
assuming  the  measure  of  the  crushing  force  at  1000. 


DESCRIPTION  OF  MATERIAL. 

Crashing  force  per 
square  inch. 

Mean  tensile  force 
per  square  inch. 

Mean  transverse  force 
of  a  bar  1  inch  square 
and  1  foot  long. 

Timber       .... 
Cast  iron    .... 

Glass,  (plate  and  crown) 

1000 
1000 
1000 
1000 

1900 
158 
100 
123 

85.1 
19.8 
9.8 
10 

281.  Strength  of  Stone.  The  marked  difference  in  the 
structure,  and  in  the  proportions  of  the  component  elements  fre- 
quently observed  in  stone  from  the  same  quarry,  would  lead  to 
the  conclusion  that  corresponding  variations  would  be  found  in 
the  strength  of  stones  belonging  to  the  same  class  ;  a  conclusion 
which  experiment  has  confirmed.  The  experiments  made  by 
different  individuals  on  this  subject,  from  not  having  been  con- 
ducted in  the  same  manner,  and  from  the  omission  in  most  cases 
of  details  respecting  the  structure  and  component  elements  of  the 
materia,  tried,  have,  in  some  instances,  led  to  contradictory  re 
suits.  A  few  facts,  however,  of  a  general  character  have  been 
ascertained,  which  may  serve  as  guides  in  ordinary  cases  ;  bul 
in  important  structures,  where  heavy  pressures  are  to  be  sus- 
tained, direct  experiment  is  the  only  safe  course  for  the  engineer 
to  follow,  in  selecting  a  material  from  untried  quarries. 


STRENGTH  OF  MATERIALS. 


71 


282.  Owing  to  the  ease  with  which  stones  generally  break 
under  a  percussive  force,  and  from  the  comparatively  slight  re- 
sistance they  offer  to  a  tensile  force,  and  to  a  transverse  strain, 
they  are  seldom  submitted  in  structures  to  any  other  strain  than 
one  of  compression ;  and  cases  but  rarely  occur  where  this  strain 
is  not  greatly  beneath  that  which  the  better  class  of  building 
stones  can  sustain  permanently,  without  undergoing  any  change 
in  their  physical  properties.  Where  the  durability  of  the  stone, 
therefore,  is  well  ascertained,  it  may  be  safely  used  without  a  re 
sort  to  any  specific  experiment  upon  its  strength,  whenever,  in 
its  structure  and  general  appearance,  it  resembles  a  material  of 
the  same  class  known  to  be  good. 

283.  The  following  Table  exhibits  tha. principal  results  of  ex- 
periments made  by  Mr.  G.  Rennie,  and  published  in  the  Philo- 
sophical Transactions  of  1818.  The  stones  tried  were  in  small 
cubes,  measuring  one  and  a  half  inches  on  the  edge.  The  table 
gives  the  pressure,  in  tons,  borne  by  each  superficial  inch  of  the 
stone  at  the  moment  of  crushing. 


— -  --               -      ■'-'   ' 

DESCRIPTION  OF  STOKE. 

Spec,  gravity. 

Crushing  w'ght 

Granites. 

2.625 
2.662 

4.83 
3.70 
2.83 

Sandstones. 

Do.             

Derby,  (red  and  friable)          .... 

2.530 
2.506 
2.316 

2.96 
2.70 
1.40 

Lime-stones. 

Marble,  (white-veined  Italian) 

Do.       (white  Brabant)           .... 
Limerick,  (black  compact)        .... 
Devonshire,  (red  marble)         .... 
Portland  stone,  (fine-grained  oolite) 

2.726 
2.697 
2.598 

2.428 

4.32 
4.11 
3.95 
3.31 

2.04 

The  following  results  are  taken  from  a  series  of  experiments 
made  under  the  direction  of  Messrs.  Bramah  &  Sons,  and  pub- 
lished in  Vol.  1,  Transactions  of  the  Institution  of  Civil  En- 
gineers. The  first  column  of  numbers  gives  the  weights,  in  tons, 
borne  by  each  superficial  inch  when  the  stones  commenced  to 
fracture  ;  the  second  column  gives  the  crushing  weight,  in  tons, 
on  the  same  surface. 


72 


BUILDING  MATERIAL. 


DESCRIPTION  OF  STONE. 

Aver,  weight  pro- 
ducing fractures. 

—     -         — i 

Average  crushing 

weight 

Granites. 

Herme    .         .         .         .         .         . 

4.77 

6.64 

Aberdeen,  (blue) 

4.13 

4.64 

Heytor    ....... 

3.94 

6.19 

Dartmoor 

3.52 

5.48 

Peterhead,  (red) 

2.88 

4.88 

Peterhead,  (blue  gray)     .... 

2.86 

1.36 

Sand-stones. 

Yorkshire        .... 

2.87 

3.94 

Craigleith        ...... 

1.89 

2.97 

Humbic 

1.69 

2.06 

Whitby 

1.00 

1.06 

1 

The  following  Table  is  taken  from  one  published  in  Vol.  2, 
Civil  Engineer  and  Architect's  Journal,  which  forms  a  part  of 
the  Report  on  the  subject  of  selecting  stone  for  the  New  Houses 
of  Parliament.  The  specimens  submitted  to  experiment  were 
cubical  blocks  measuring  two  inches  on  an  edge. 


DKSCR1PTIOK  Or  STONB. 

Specific  gravity. 

Weight  produ- 
cing fracture. 

Crushing  w'ght. 

Sand-stones. 

Craigleith           .... 
Darley  Dale      .... 

Heddon 

Kenton 

Mansfield  .... 

2.232 
2.628 
2.229 
2.247 
2.338 

1.89 
2.75 
0.82 
1.51 

0.88 

3.5 

3.1 

1.75 

2.21 

1.64 

Magnesian  Lime-stones. 

Bolsover 

Huddlestone                . 

Roach  Abbey     .... 

Park  Nook        .... 

2.316 
2.147 
2.134 
2.138 

2.21 
1.03 
0.75 
0.32 

3.75 

1.92 
1.73 
1.92 

Oolites. 

Ancaster  ... 

Bath  Box 

Portland 

Ketton 

2.182 
1.839 
2.145 
2.045 

0.75 
0.56 
0.95 
0.69 

1.04 
0.66 
1.75 
1.18 

Limestones. 

Barnack 

Chilmark,  (si.iaous) 

Hamhill 

2.090 
2.481 
2.260 

0.50 
1.32 
0.69 

0.79 

3.19        i 
1.80 

The  numbers  of  the  first  column  give  the  specific  gravities 


STRENGTH  OF  MATERIALS. 


72 


those  in  the  second  column  the  weight  in  tons  on  a  s^  lare  inch, 
when  the  stone  commenced  to  fracture ;  and  those  in  the  third 
the  crushing  weight  on  a  square  inch. 

The  following  Table  exhi  tits  the  results  of  experiments  on  the 
resistance  of  stone  to  a  transverse  strain,  made  by  Colonel  Pasley, 
on  prisms  4  inches  long,  the  cross  section  being  a  square  of  2 
inches  on  a  side ;  the  distance  between  the  points  of  support 
3  inches. 


DESCRIPTION   OF    STONE. 

Weight  of  stone 
per   cubic  foot 
in  lbs. 

Average  breaking 
weight  in  lbs. 

1.  Kentish  Rag 

2.  Yorkshire  Landing 

3.  Cornish  granite    . 

4.  Portland       .... 

5.  Craigleith    ...... 

6.  Bath 

7.  Well-burned  bricks       .... 

8.  Inferior  bricks      ..... 

165.69 
147.67 
172.24 
148.08 
144.47 
122.58 
91.71 

4581 

2887 

2808 

2682 

1896 

666 

752 

329 

284.  The  conductors  of  the  experiments  on  the  stone  for  the 
New  Houses  of  Parliament,  Messrs.  Daniell  and  Wheatstone, 
who  also  made  a  chemical  analysis  of  the  stones,  and  applied  to 
them  Brard's  process  for  testing  their  resistance  to  frost,  have 
appended  the  following  conclusions  from  their  experiments : — 
"  If  the  stones  be  divided  into  classes,  according  to  their  chemical 
composition,  it  will  be  found  that  in  all  stones  of  the  same  class 
there  exists  generally  a  close  relation  between  their  various  phy- 
sical qualities.  Thus  it  will  be  observed  that  the  specimen  which 
has  the  greatest  specific  gravity  possesses  the  greatest  cohesive 
strength,  absorbs  the  least  quantity  of  water,  and  disintegrates 
the  least  by  the  process  which  imitates  the  effects  of  weather. 
•1  comparison  of  all  the  experiments  shows  this  to  be  the  general 
rule,  though  it  is  liable  to  individual  exceptions." 

"  But  this  will  not  enable  us  to  compare  stones  of  different 
classes  together.  The  sand-stones  absorb  the  least  quantity  of 
water,  but  they  disintegrate  more  than  the  magnesian  lime-stones, 
which,  considering  their  compactness,  absorb  a  great  deal." 

285.  Rondelet,  from  a  numerous  series  of  experiments  on  the 
same  subject,  published  in  his  work,  Art  de  Batir,  has  arrived 
at  like  conclusions  with  regard  to  the  relations  between  the 
specific  gravity  and  strength  of  stones  belonging  to  the  same 
class. 

2S6.  Among  the  results  of  the  more  recent  experiments  on  this 
subject,  those  obtained  by  Mr.  Hodgkinson,  showing  the  relation 

10 


74  BUILDING  MATERIALS. 

between  the  crushing,  the  tensile,  and  the  transverse  strength  of 
stone,  have  already  been  given. 

M.  Vicat,  in  a  memoir  on  the  same  subject,  published  in  the 
Annates  des  Ponts  et  CJiaussees,  1 833,  has  arrived  at  an  opposite 
conclusion  from  Mr.  Hodgkinson,  stating,  as  the  results  of  his 
experiments,  that  no  constant  relation  exists  between  the  crush- 
ing and  tensile  strength  of  stone  in  general,  and  that  there  is  no 
other  means  of  determining  these  two  forces,  but  by  direct  ex- 
periment in  each  case. 

287.  The  influence  of  form  on  the  strength  of  stone,  and  the 
circumstances  attending  the  rupture  of  hard  and  soft  stones,  have 
been  made  the  subject  of  particular  experiments  by  Rondelet  and 
Vicat.  Their  experiments  agree  in  establishing  the  points  that 
the  crushing  weight  is  in  proportion  to  the  area  of  the  base. 
Vicat  states,  more  generally,  that  the  permanent  weights  borne 
by  similar  solids  of  stone,  under  like  circumstances,  will  be  as 
the  squares  of  their  homologous  sides.  These  two  authors  agree 
on  the  point  that  the  circular  form  of  the  base  is  the  most  favor- 
able to  strength.  They  differ  on  most  other  points,  and  particu- 
larly on  the  manner  in  which  the  different  kinds  of  stone  yield  by 
rupture. 

288.  Practical  Deductions.  Were  stones  placed  under  the 
same  circumstances  in  structures  as  in  the  experiments  made  to 
ascertain  their  strength,  there  would  be  no  difficulty  in  assigning 
what  fractional  part  of  the  weight  which,  in  the  comparatively 
short  period  usually  given  to  an  experiment,  will  crush  them, 
could  be  borne  by  them  permanently  with  safety.  But,  in- 
dependently of  the  accidental  causes  of  destruction  to  whicl 
structures  are  exposed,  imperfections  in  the  material  itself,  as 
well  as  careless  workmanship,  from  which  it  is  often  placed 
in  the  most  unfavorable  circumstances  of  resistance,  require  to 
be  guarded  against.  M  Vicat,  in  the  memoir  before-mentioned, 
states  that  a  permanent  .strain  of  TVV  °f  the  crushing  force  of  ex- 
periment, may  be  borne  by  stone  without  danger  of  impairing  its 
cohesive  strength,  provided  it  be  placed  under  the  most  favorable 
circumstances  of  resistance.  This  fraction  of  the  crushing  weight 
of  experiment  is  greater  than  ordinary  circumstances  would  jus- 
tify, and  it  is  recommended  in  practice  not  to  submit  any  stone 
'o  a  greater  permanent  strain  than  one  tenth  of  the  crushing  weight 
of  experiments  made  on  small  cubes  measuring  about  two  inches 
on  an  edge. 

The  following  Table  shows  the  permanent  strain,  and  crushing 
weight,  for  a  square  foot  of  the  stones  in  some  of  the  most  re 
markable  structures  in  Europe. 


STRENGTH  OF  MATERIALS. 


75 


Pillars  of  the  dome  of  St.  Peter's,  (Rome) 
Do.  St.  Paul's,  (London) 

Do.  St.  Genevieve,  (Paris) 

Do.       Church  of  Toussaint,  (Angers)  . 

Lower  courses  of  the  piers  of  the  Bridge  of  Neuilly 


Permanent 

Crushing 

strain. 

weight. 

33330 

536000 

39450 

537000 

60000 

456000 

90000 

900000 

3600 

570000 

The  stone  employed  in  all  the  structures  enumerated  in  the 
Table,  is  some  variety  of  lime-stone. 

289.  Eccpansion  of  Stone  from  Heat.  Experiments  have  been 
made  in  this  country  by  Prof.  Bartlett,  and  in  England  by  Mr. 
Adie,  to  ascertain  the  expansion  of  stone  for  every  degree  of 
Fahrenheit.  The  experiments  of  Prof.  Bartlett  give  the  follow- 
ing results  : 


Granite  expands  for  every  degree 
Marble  "  " 

Sand-stone        "  " 


.000004825 
.000005668 
.000009532 


Table  of  the  Expansion  of  Stone,  SfC.,from  the  Experiments  oj 
Alexander  J.  Adie,  Civil  Engineer,  Edinburgh. 


DESCRIPTION  OF  STONE. 


1.  Roman  cement . 

2.  Sicilian  white  marble   . 

3.  Carrara  marble 

4.  Sdnd-stone,  (Craigleith) 

5.  Slate,  ( Welch)  .      .      . 

6.  Red  granite,  (Peterhead) 

7.  Arbroath  pavement 

8.  Caithness  pavement 

9.  Green-stone,  (Ratho)     . 

10.  Gray  granite,  (Aberdeen) 

11.  Be>U  stock  brick 

12.  Fire  brick    .... 

13.  Black  marble,  (Oalway) 


Decimal  of 
inch  ca    ti 
inches 
180'F. 


.0330043 

.0325392 

.0253946 

.0274344 

.0150405 

.0270093 

.0238659 

.0220416 

.0206261) 

.0206652 

.0205788 

.0186043 

.01815695 

.0126542 

.0113334 

.0102394 


Decimal    of 
length   for 

180-  y. 


.0014349 

.0014147 

.00110411 

.0011928 

.00065:19 

.0011743 

.0010376 

.0009583 

.0008968 

.0008085 

.0008947 

.0008099 

.00078943 

.0005502 

.0004928 

.00044519 


Decimal  of 
length  for 
1»F. 


.00000750 
.00000780 
.00000613 
.00000662 
.00000:i63 
.00000652 
.00000576 
.00000532 
.00000498 
.0O0OT1499 
.00000497 
.00000449 
.00000438 
.00000306 
.00000274 
.00000247 


One  experiment,  (moist.) 
Mean  of  three,  (dry.) 
One  experiment,  (vioist.) 
Mean  of  two.  (dry.) 
Mean  of  four  experiments. 
Mean  of  three        do. 
One  experiment,  (moist.) 
Mean  of  two,  (dry.) 
Mean  of  four  experiments. 
Mean  of  three        do. 
Mean  of  three        do 
Mean  of  two  do. 

Mean  of  two  do. 

Mean  of  two  do. 

Mean  of  three        do. 


290.  Strength  of  Mortars.  A  very  wide  range  of  experi- 
ments has  been  made,  within  a  few  years  back,  by  engineers  both 
at  home  and  abroad,  upon  the  resistance  offered  by  mortars  to  a 
transversal  strain,  with  a  view  to  compare  their  qualities,  both  as 
regards  their  constituent  elements  and  the  processes  followed  in 
their  manipulation.  As  might  naturally  have  been  anticipated 
these  experiments  have  presented  very  diversified,  and,  in  many 
instances,  contradictory  results.  The  general  conclusions,  how- 
ever, drawn  from  them,  have  been  nearly  the  same  in  the  majority 


76  BUILDING  MATERIALS. 

of  cases ;  and  they  furnish  the  engineer  with  the  most  reliable 
guides  in  this  important  branch  of  his  art. 

291.  The  usual  method  of  conducting  these  experiments  has 
been  to  subject  small  rectangular  prisms  of  mortar,  resting  on 
points  of  support  at  their  extremities,  to  a  transversal  strain  ap- 
plied at  the  centre  point  between  the  bearings.  This,  perhaps, 
is  as  unexceptionable  and  convenient  a  method  as  can  be  followed 
for  testing  the  comparative  strength  of  mortars. 

292.  M.  Vicat,  in  the  work  already  cited,  gives  the  following 
as  the  average  resistances  on  the  square  inch  offered  by  mortars 
to  a  force  of  traction ;  the  deductions  being  drawn  from  experi- 
ments on  the  resistance  to  a  transversal  strain. 

Mortars  of  very  strong  hydraulic  lime  .     170  pounds 

"  ordinary  do.  .140      " 

"  medium  do.  .     100      " 

"  common  lime  .  40      " 

"  do.  (bad  quality)  .       10      " 

These  experiments  were  made  upon  prisms  a  year  old,  which 
had  been  exposed  to  the  ordinary  changes  of  weather.  With  re- 
gard to  the  best  hydraulic  mortars  of  the  same  age  which  had 
been,  during  that  same  period,  either  immersed  in  water,  or 
buried  in  a  damp  position,  M.  Vicat  states  that  their  average 
tenacity  may  be  estimated  at  140  pounds  on  the  square  inch. 

293.  General  Treussart,  in  his  work  on  hydraulic  and  common 
mortars,  has  given  in  detail  a  large  number  of  experiments  on  the 
transversal  strength  of  artificial  hydraulic  mortars,  made  by  sub- 
mitting small  rectangular  parallelopipeds  of  mortar  six  inches  in 
length,  and  two  inches  square,  to  a  transversal  strain  applied  at 
the  centre  point  between  the  bearings,  which  were  four  inches 
apart.  From  these  experiments  he  deduces  the  following  prac- 
tical conclusions. 

That  when  the  parallelopipeds  sustain  a  transversal  strain  vary- 
ing between  220  and  330  pounds,  the  corresponding  mortar  will 
be  suitable  for  common  gross  masonry ;  but  that  for  important 
hydraulic  works  the  parallelopipeds  should  sustain,  before  yield  ' 
ing,  from  330  to  440  pounds. 

294.  The  only  published  experiments  on  this  subject  made  in 
this  country  are  those  of  Colonel  Totten,  appended  to  his  transla- 
tion of  General  Treussart's  work.  The  results  of  these  experi- 
ments are  of  peculiar  value  to  the  American  engineer,  as  they 
were  made  upon  materials  in  very  general  use  on  the  public 
works  throughout  the  country. 

From  these  experiments  Colonel  Totten  deduces  the  following 
general  results  : 

1st.  That  mortar  of  hydraulic  cement  and  sand  is  the  strongei 
and  harder  as  the  quantity  of  sand  is  less. 


STRENGTH  OF  MATERIALS.  77 

2d.  That  common  mortar  is  the  stronger  and  harder  as  the 
quantity  of  sand  is  less. 

3d.  That  any  addition  of  common  lime  to  a  mortar  of  hydraulic 
cement  and  sand  weakens  the  mortar,  but  that  a  little  lime  ma\ 
be  added  without  any  considerable  diminution  of  the  strength  of 
the  mortar,  and  with  a  saving  of  expense. 

4th.  The  strength  of  common  mortars  is  considerably  improved 
by  the  addition  of  an  artificial  puzzolana,  but  more  so  by  the  ad- 
dition of  an  hydraulic  cement. 

5th.  Fine  sand  generally  gives  a  stronger  mortar  than  coarse 
sand. 

6th.  Lime  slaked  by  sprinkling  gave  better  results  than  lime 
slaked  by  drowning.  A  few  experiments  made  on  air-slaked  lime 
were  unfavorable  to  that  mode  of  slaking. 

7th.  Both  hydraulic  and  common  mortar  yielded  better  results 
when  made  with  a  small  quantity  of  water  than  when  made  thin. 

8th.  Mortar  made  in  the  mortar-mill  was  found  to  be  superior 
to  that  mixed  in  the  usual  way  with  a  hoe. 

9th.  Fresh  water  gave  better  results  than  salt  water. 

295.  Strength  of  Concrete  and  Beton.  From  experiments 
made  on  concrete,  prepared  according  to  the  most  approved  pro- 
cess in  England,  by  Colonel  Pasley,  it  appears  that  this  material 
is  very  inferior  in  strength  to  good  brick,  and  the  weaker  kinds 
of  natural  stones. 

From  experiments  made  by  Colonel  Totten  on  beton,  the  fol- 
lowing conclusions  are  drawn  : 

That  beton  made  of  a  mortar  composed  of  hydraulic  cement, 
common  lime,  and  sand,  or  of  a  mortar  of  hydraulic  cement  and 
sand,  without  lime,  was  the  stronger  as  the  quantity  of  sand  was 
the  smaller.  But  there  may  be  0.50  of  sand,  and  0.25  of  com- 
mon lime,  without  sensible  deterioration ;  and  as  much  as  1 .00  of 
sand,  and  0.25  of  lime,  without  great  loss  of  strength. 

Beton  made  with  just  sufficient  mortar  to  fill  the  void  spaces 
between  the  fragments  of  stone  was  found  to  be  less  strong  than 
that  made  with  double  this  bulk  of  mortar.  But  Colonel  Totten 
remarks,  that  this  result  is  perhaps  attributable  to  the  difficulty 
of  causing  so  small  a  quantity  of  mortar  to  penetrate  the  voids, 
and  unite  all  the  fragments  perfectly,  in  experiments  made  on  a 
small  scale. 

The  strongest  beton  was  obtained  by  using  quite  small  frag- 
ments of  brick,  and  the  weakest  from  small,  rounded,  stone  gravel. 

A  beton  formed  by  pouring  grout  among  fragments  of  stone,  oi 
brick,  was  inferior  in  strength  to  that  made  in  the  usual  way  with 
mortar. 

Comparing  the  strength  of  the  betons  on  which  the  experi- 
ments were  made,  which  were  eight  months  old  when  tried,  with 


78 


BUILDING  MATERIALS. 


that  of  a  sample  of  sound  red  sand  stone  of  good  quality,  it  ap 
pears  that  the  strongest  prisms  of  bcton  were  only  half  as  strong 
as  the  sand-stone. 

296.  Strength  of  Timber.  A  wide  range  of  experiments 
has  been  made  on  the  resistance  of  timber  to  compression,  ex- 
tension, and  a  transverse  strain,  presenting  very  variable  results 
Among  the  most  recent,  and  which  command  the  greatest  confi- 
dence from  the  ability  of  their  authors,  are  those  of  Professor 
Barlow  and  Mr.  Hodgkinson :  the  former  on  the  resistance  to 
extension  and  a  transverse  strain ;  the  latter  on  that  to  com- 
pression. 

297.  Resistance  to  Extension.  The  following  Table  exhibits 
the  specific  gravity,  and  the  mean  resistance  per  square  inch  of 
various  kinds  of  timber,  from  the  experiments  of  Prof.  Barlow. 


DESCRIPTION  OF  TIMBER. 


Ash,  (English) 
Beech,     do. 
Box      . 

Deal,  (Christiana) 
Do.   (Meniel 
Elm      . 

Fir,  (New  England) 
Do.  (Riga)    . 
Do.  (Mar  Forest) 
Larch,  (Scotch)     . 
Locust 
Mahogany     . 
Norway  spars 

Oak,  (English) 

Do.    (African) 
Do.    (Adriatic) 
Do.    (Canadian     . 
Do.    '(Dantzic) 
Pear     . 
Poon     . 
Pine,  (pitch) 
Do.     (red)    . 
Teak    . 


from 
to 


Spec.  grav. 


0.76C 
0.7^/0 
1.000 
C.680 
0.590 
0.540 
0.550 
0.750 
0.700 
0.540 
0.950 
0.637 
0.580 
0.700 
0.900 
0.980 
0.990 
0.872 
0.760 
0.646 
0.600 
0.660 
0.660 
0.750 


Mean  strength  of 
cohesion    per 
square  inch. 


17000 
11500 
20000 
11000 
11000 

5780 
12000 
12600 
12000 

7000 
20580 

8000 
12000 

9000 
15000 
14400 
14000 
12000 
14500 

9800 
14000 
10500 
10000 
15000 


298.  But  few  direct  experiments  have  been  made  upon  the 
elongations  of  timber  from  a  strain  in  the  direction  of  the  fibres 
From  some  made  in  France  by  MM.  Minard  and  Desormes,  it 
would  appear  that  bars  of  oak  having  a  sectional  area  of  one 
square  rich,  will  be  elongated  .001176  of  their  length  by  a  strain 
of  ow  ton. 


STRENGTH  OF  MATERIALS. 


7S 


299.  Resistance  to  Compression.  The  following  Table  ex- 
hibits the  results  obtained  by  Mr.  Hodgkinson  from  exr.  erimenta 
on  short  cylinders  of  timber  with  flat  ends.  The  diameter  of 
each  cylinder  was  one  inch,  and  its  height  two  inches.  The  re- 
sults, in  the  first  column,  are  a  mean  from  about  three  experiments 
on  timber  moderately  dry,  being  such  as  is  used  for  making 
models  for  castings  ;  those  in  the  second  column  were  obtained, 
in  a  like  manner,  from  similar  specimens,  which  were  turned  and 
kept  dry  in  a  warm  place  two  months  longer.  A  comparison  of 
the  results  in  the  two  columns,  shows  the  effect  of  drying  on  the 
strength  of  timber ;  wet  timber  not  having  half  the  strength  of 
the  same  when  dry.  The  circumstances  of  rupture  were  the 
same  as  already  stated  in  the  general  observations  under  this 
head ;  the  height  of  the  wedge  which  would  slide  off  in  tim- 
ber being  about  half  the  diameter,  or  thickness  of  the  specimen 
crushed. 


DiscRimos  or  wood. 


Alder  

Ash 

Baywood      .... 
Beech  .... 

Birch,  {American) 

Do.    {English)  . 
Cedar  .... 

Crab 

Red  deal      .... 
White  deal  .... 

Elder  

Elm 

Fir,  {spruce) 

Hornbeam   .... 

Mahogany    .... 

Oak,  {Quebec) 

Do.    {English)     . 

Do.    {Dantzic,  very  dry) 

Pine,  {pitch) 

Do.     (yellow,  full  nf  turpentine) 

Do.     {red)  ... 

Poplar  .... 

Plum,  {wet) 

Do.     {dry) 
Sycamore    .... 

Teak 

Larch,  {fallen  two  months)  . 

Walnut         .         . 

Willow         .... 


Strength  per  square  inch 

in  lbs. 

6831 

6960 

8683 

9363 

7518 

7518 

7733 

19363 

3297 

11663 

3297 

6402 

5674 

5863 

6499 

7148 

5748 

6586 

6781 

7293 

7451 

9973 

- 

10331 

6499 

6819 

4533 

7289 

8198 

8198 

4231 

5982 

6484 

10058 

- 

7731 

6790 

6790 

5375 

5445 

5395 

7518 

3107 

5124 

3654 

- 

8241 

to  1049 

7082 

- 

- 

12101 

3201 

5568 

6063 

7227 

2898 

6128 

300.   Resistance  of  Square  Pillars.     Mr.  Hodgkinscn  has 


80  BUILDING  MATERIALS. 

made  a  number  of  invaluable  experiments  on  the  strengtn  of 
pillars  of  timber,  and  of  columns  of  iron  and  steel,  and  from 
them  has  deduced  formulae  for  calculating  the  pressure  which 
they  will  support  before  breaking.  The  experiments  on  timber 
were  made  on  pillars  with  flat  ends.  The  following  are  the  for- 
mulae from  which  their  strength  may  be  estimated. 

Calling  the  breaking  weight  in  lbs.    W. 

"      the  side  of  the  square  base  in  inches  d. 
"      the  length  of  the  pillar  in  feet  /. 

Then  for  long  columns  of  oak,  in  which  the  side  of  the  squai 
base  is  less  than  gVth  the  height  of  the  column  ; 

W=  24542-^-. 
and  for  red  deal, 

W=\lb\\j. 

For  shorter  pillars,  where  the  ratio  between  their  thickness  and 
height  is  such  that  they  still  yield  by  bending,  the  strength  is  es- 
timated by  the  following  formula  : 

Calling  the  weight  calculated  from  either  of  the  preceding  for- 
mulae, W. 

Calling  the  crushing  weight,  as  estimated  from  the  preceding 
Table,  W. 

Calling  the  breaking  weight  in  lbs.,  W". 

Then  the  formula  for  the  strength  is 

WW' 

W"  = 

w+$w 

In  each  of  the  preceding  formula3  d  must  be  taken  in  inches, 
and  I  in  feet. 

301.  Resistance  to  Transverse  Strains.  As  timber,  from  the 
purposes  to  which  it  is  applied,  is  for  the  most  part  exposed  to  a 
transverse  strain,  the  far  greater  number  of  experiments  have  been 
made  to  ascertain  the  relations  between  the  strain,  the  deflection 
caused  by  it,  and  the  linear  dimensions  of  the  piece  subjected 
to  the  strain.  These  relations  have  been  made  the  subject  of 
mathematical  investigations,  founded  upon  data  derived  from  ex- 
periment, which  will  be  given  in  the  Appendix.  The  following 
Table  exhibits  the  results  of  experiments  made  upon  beams  having 
a  rectangular  sectional  area,  which  were  laid  horizontally  upon 
supports  at  their  ends,  and  subjected  to  a  strain  applied  at  the 
middle  point  between  the  supports,  in  a  vertical  direction. 

For  a  more  convenient  application  of  the  formulae  to  the  results 
of  the  experiments,  the  notation  adopted  in  the  preceding  Art 
will  be  here  given. 


STRENGTH  OF  MATERIALS. 


81 


Ca.l  the  transverse  force  necessary  to  break  th  z  beam  in  lbs.,  W 
"    the  distance  between  the  supports  in  inches,  I. 
"    the  horizontal  breadth  of  the  sectional  area  in  inches,  b. 
"    the  vertical  depth  "  "  "         d. 

"    the  deflection  arising  from  a  weight  w  in  inches,/. 

Table  of  Experiments  with  the  foregoing  Notation. 


■ 

DESCRIPTION  Or  WOOD. 

Specific 
grav. 

Values  Values 
of     1     of 
I.           b. 

Value 
of 
d. 

Value 
of 

Value 
of 
10. 

Value 
of 

w. 

Authors   of   ex. 

per  intents. 

Inches. 

Inches. 

Inches. 

Inches. 

it. ■. 

lb.. 

'  Oak,  (English)    .      .      . 

.934 

84 

2 

2 

1.280 

200 

637 

Prof.  Barlow. 

Do.     (Canadian) 

.872 

84 

2 

2 

1.080 

225 

673 

" 

Pine,  (American) 

- 

84 

2 

2 

0.931 

150 

- 

" 

Oak,  (English]   .       .      . 

- 

30 

1 

1 

0.5 

137 

- 

Tredgold. 

White  spruce,  (Canadian) 

.465 

24 

1 

1 

0.5 

180 

285 

" 

White  pine,  (American)  . 

.455 

85.2 

2.75 

5.55 

0.177 

777 

5189 

Lieut.  Brown. 

Black  spruce,      do. 

.400 

85.2 

2.75 

5.55 

0.177 

892 

5646 

" 

Southern  pine,    do. 

.872 

85.2 

2.75 

5.54 

0.177 

1175 

9237 

302.  Resistance  to  Detrusion.  From  the  experiments  of  Prof. 
Barlow,  it  appears  that  the  resistance  offered  by  the  lateral  adhe- 
sion of  the  fibres  of  fir,  to  a  force  acting  in  a  direction  parallel  to 
the  fibres,  may  be  estimated  at  592  lbs.  per  square  inch. 

Mr.  Tredgold  gives  the  following  as  the  results  of  experiments 
on  the  resistance  offered  by  adhesion  to  a  force  applied  perpen- 
dicularly to  the  fibres  to  tear  them  asunder. 

Oak      .       .     2316  lbs.  per  square  inch. 
Poplar  .       .1782       "  " 

Larch,  970  to  1700      "  " 

303.  Strength  of  Cast  Iron.  The  most  recent  experiments 
on  the"  strength  of  this  material  are  those  of  Mr.  Hodgkinson. 
Those,  particularly,  made  by  him  on  the  subject  of  the  strength 
of  columns,  and  the  most  suitable  form  of  cast-iron  beams  to  sus- 
tain a  transversal  strain,  have  supplied  the  engineer  and  architect 
with  the  most  valuable  guide  in  adapting  this  material  to  the 
various  purposes  of  structures. 

304.  Resistance  to  Extension.  From  a  few  experiments  made 
by  Mr.  Rennie  and  Captain  Brown,  the  tensile  strength  of  cast 
iron  varies  from  7  to  9  tons  per  square  inch. 

The  experiments  of  Mr.  Hodgkinson  upon  both  hot  and  cold 
blast  iron  give  the  tensile  strength  from  6  to  9£  tons  per  square 
inch. 

From  some  experiments  made  on  American  cast  iron,  under 
the  direction  of  the  Franklin  Institute,  the  mean  tensile  strength 
is  20834  lbs.,  or  9|  tons  per  square  inch. 

305.  Resistance  to  Compression.  The  general  circumstances 
attending  the  rupture  of  this  material  by  compression,  drawn  frono 

11 


82 


BUILDING  MATERIALS. 


he  experiments  of  Mr.  Hodgkinson,  have  already  been  given 
The  angle  of  the  wedge  resulting  from  the  rupture  is  about  55°. 

The  mean  crushing  weight  derived  from  experiments  upor 
short  cylinders  of  hot  blast  iron  was  121,685  lbs.,  or  54  tons  6| 
cwt.  per  square  inch. 

That  on  short  prisms  of  the  same,  with  square  bases,  100,738 
lbs.,  or  44  tons  19 j  cwt.  per  square  inch. 

That  on  short  cylinders  of  cold  blast  iron  was  1 25,403  lbs.,  oi 
55  tons  19^  cwt.  per  square  inch. 

That  on  short  prisms  of  the  same,  having  other  regular  figures 
for  their  bases,  was  100,631  lbs.,  or  44  tons  18|  cwt.  per  square 
inch. 

Mr.  Hodgkinson  remarks  with  respect  to  the  forms  of  base 
differing  from  the  circle  :  "In  the  other  forms  the  difference  of 
strength  is  but  little  ;  and  therefore  we  may  perhaps  admit  that 
difference  of  form  of  section  has  no  influence  upon  the  power  of 
a.  short  prism  to  bear  a  crushing  force." 

In  remarking  on  the  circumstances  attending  the  rupture,  Mr. 
Hodgkinson  farther  observes :  "  We  may  assume,  therefore, 
without  assignable  error,  that  in  the  crushing  of  short  iron  prisms 
of  various  forms,  longer  than  the  wedge,  the  angle  of  fracture  will 
be  the  same.  This  simple  assumption,  if  admitted,  would  prove 
at  once,  not  only  in  this  material,  but  in  others  which  break  in  the 
same  manner,  the  proportionality  of  the  crushing  force  in  different 
forms  to  the  area ;  since  the  area  of  fracture  would  always  be 
equal  to  the  direct  transverse  area  multiplied  by  a  constant  quan- 
tity dependent  upon  the  material." 


Table  exhibiting  the  Ratio  of  the  Tensile  to  the  Compressive 
Forces  in  Cast  Iron,  from  Mr.  Hodgkinson 's  Experiments. 


DESCRIPTION  OF  METAL. 

Compressive  force 
per  square  inch. 

Tensile  force  per 
square  inch. 

Ratio. 

Devon  iron, 

No.  3.  Hot  blast 

145,435 

21,907 

6.638  :  1 

Buffery  iron, 

No.  1.  Hot  blast 

86,397 

13,434 

6.431  :  1 

Do. 

"      Cold  blast 

33,385 

17,466 

5.346  :  1 

Coed-Talen  iron 

,No.  2.  Hot  blast 

82,734 

16,676 

4.961  :  1 

Do. 

"      Cold  blast 

81,770 

18,855 

4.337  :  1 

Carron  iron, 

No.  2.  Hot  blast 

108,540 

13,505 

8.037  :  1 

Do. 

"      Cold  blast 

106,375 

16,683 

6.376  :  1 

Carron  iron, 

No.  3.  Hot  blast 

133,440 

17,755 

7.515  :  1 

Do. 

"      Cold  blast 

115,442 

14,200 

8.129  :  1 

306.  Resistance  of  Cylindrical  Columns.  The  experiments 
under  this  head  were  made  upon  solid  and  hollow  columns,  both 
ends  of  which  were  either  flat  or  rounded,  fixed  or  loose,  or  one 


STRENGTH  OF  MATERIALS  83 

end  flat  and  the  other  rounded.  In  the  case  of  columns  with 
rounded  ends,  the  pressure  was  applied  in  the  direction  of  the 
axis  of  the  column. 

The  follow^g  extracts  are  made  from  Mr.  Hodgkinson's  paper 
on  this  subject,  published  in  the  Report  of  the  British  Association 
o/1840. 

"  1st.  In  all  long  pillars  of  the  same  dimensions,  the  resistance 
to  crushing  by  flexure  is  about  three  times  greater  when  the  ends 
of  the  pillars  are  flat,  than  when  they  are  rounded. 

"  2d.  The  strength  of  a  pillar,  with  one  end  rounded  and  the 
other  flat,  is  the  arithmetical  mean  between  that  of  a  pillar  of  the 
same  dimensions  with  both  ends  round,  and  one  with  both  ends 
flat.  Thus,  of  three  cylindrical  pillars,  all  of  the  same  length 
and  diameter,  the  first  having  both  its  ends  rounded,  the  second 
with  one  end  rounded  and  one  flat,  and  the  third  with  both  ends 
flat,  the  strengths  are  as  1,  2,  3,  nearly. 

"3d.  A  long,  uniform,  cast-iron  pillar,  with  its  ends  firmly 
fixed,  whether  by  means  of  discs  or  otherwise,  has  the  same 
power  to  resist  breaking  as  a  pillar  of  the  same  diameter,  and 
half  the  length,  with  the  ends  rounded  or  turned  so  that  the  force 
would  pass  through  the  axis. 

"  4th.  The  experiments  show  that  some  additional  strength  is 
given  to  a  pillar  by  enlarging  its  diameter  in  the  middle  part ;  this 
increase  does  not,  however,  appear  to  be  more  than  one  seventh, 
or  one  eighth  of  the  breaking  weight. 

"  5th.  The  index  of  the  power  of  the  diameter  to  which  the 
strength  of  long  pillars  with  rounded  ends  is  proportional,  is  3.76 
nearly,  and  3.55  in  those  with  flat  ends,  as  appeared  from  the  re- 
sults of  a  great  number  of  experiments  ;  or  the.  strength  of  both 
may  be  taken  as  the  3.6  power  of  the  diameter  nearly. 

"  6th.  In  pillars  of  the  same  thickness,  the  strength  is  inversely 
proportional  to  the  1 .7  power  of  the  length  nearly. 

"  Thus  the  strength  of  a  solid  pillar  with  rounded  ends,  the 

diameter  of  which  is  d,  and  the  length  I,  is  as  -=^." 

"  The  absolute  strength  of  solid  pillars,  as  appeared  from  the 
experiments,  are  nearly  as  below. 
In  pillars  with  rounded  ends, 

Strength  in  tons  =  14.9  ■«•. 

In  pillars  with  flat  ends, 

^3.6 

Strength  in  tons  =44.16  -jpj- 

In  hollow  pillars  nearly  the  same  laws  were  found  to  obtain ; 
hu8,  if  D  and  d  be  the  external  and  internal  diameters  of  a  pillai 


84  BUILDING  MATERIALS. 

whose  length  is  Z,  the  strength  of  a  hollow  cylinder  of  which  the 
ends  were  moveable  (as  in  the  connecting  rod  of  a  steam-engine) 
tvould  be  expressed  by  the  formula  below. 

£)'"  _  ^3-6 
Strength  in  tons  =  13 =3 . 

In  hollow  pillars,  whose  ends  are  flat,  we  had  from  experimen 

as  before, 

X)3-6  —  d3-6 
Strength  in  tons  =  44.3 ^ 

The  formulae  above  apply  to  all  pillars  whose  length  is  not 
less  than  about  thirty  times  the  external  diameter ;  for  pillars 
shorter  than  this,  it  is  necessary  to  have  recourse  to  the  '  for- 
mula,' given  under  the  head  of  Strength  of  Timber,  for  short 
pillars  of  timber,  substituting  for  W  and  W  in  that  formula,  the 
proper  values  applicable  to  cast  iron." 

307.  Similar  Pillars.  "  In  similar  pillars,  or  those  whose 
length  is  to  the  diameter  in  a  constant  proportion,  the  strength  is 
nearly  as  the  square  of  the  diameter,  or  of  any  other  linear  di- 
mension ;  or,  in  other  words,  the  strength  is  nearly  as  the  area 
of  the  transverse  section." 

"  In  hollow  pillars,  of  greater  diameter  at  one  end  than  the 
other,  or  in  the  middle  than  at  the  ends,  it  was  not  found  that 
any  additional  strength  was  obtained  over  that  of  cylindrical 
pillars." 

"  The  strength  of  a  pillar,  in  the  form  of  the  connecting  rod  of 
a  steam-engine,"  (that  is,  the  transverse  section  presenting  the 
figure  of  a  cross  +,)  "  was  found  to  be  very  small,  perhaps  not 
half  the  strength  that  the  same  metal  would  have  given  if  cast  in 
the  form  of  a  uniform  hollow  cylinder." 

"  A  pillar  irregularly  fixed,  so  that  the  pressure  would  be  in 
the  direction  of  the  diagonal,  is  reduced  to  one  third  of  its  strength. 
Pillars  fixed  at  one  end  and  moveable  at  the  other,  as  in  those  flat 
at  one  end  and  rounded  at  the  other,  break  at  one  third  the  length 
from  the  moveable  end ;  therefore,  to  economize  the  metal,  they 
should  be  rendered  stronger  there  than  in  other  parts." 

308.  Long-continued  Pressure  on  Pillars.  "  To  determine 
the  effect  of  a  load  lying  constantly  on  a  pillar,  Mr.  Fairbairn  had, 
at  the  writer's  suggestion,  four  pillars  cast,  all  of  the  same  length 
and  diameter.  The  first  was  loaded  with  4  cwt.,  the  second 
with  7  cwt.,  the  third  with  10  cwt.,  and  the  fourth  with  13  cwt. ; 
this  last  load  was  T9/^  of  what  had  previously  broken  a  pillar  of 
the  same  dimensions,  when  the  weight  was  carefully  laid  on  with- 
out loss  of  time.  The  pillar  loaded  with  1 3  cwt.  bore  the  weight 
between  five  and  six  months,  and  then  broke." 

309.  General  Properties  of  Pillars.     "  In  pillars  of  wrought 


STRENGTH  OF  MATERIALS. 


85 


iron,  steel,  and  timber,  the  same  laws,  with  respect  to  rounded 
and  flat  ends,  were  found  to  obtain,  as  had  been  shown  to  exist 
in  cast  iron."' 

"  Of  rectangular  pillars  of  timber,  it  was  proved  experimental 
y  that  the  pillar  of  greatest  strength  of  the  same  material  is  a 
square." 

310.  Comparative  Strengths  of  Cast  Iron,  Wrought  Iron, 
Steel,  and  Timber. 

"  It  resulted  from  the  experiments  upon  pillars  of  the  same 
dimensions  but  of  different  materials,  that  if  we  call  the  strength 
of  cast  iron  1000,  we  shall  have  for  wrought  iron  1745,  cast  steel 
2518,  Dantzic  oak  108.8,  red  deal  78.5." 

311.  Resistance  to  Transverse  Strains.  The  following  Tables 
and  deductions  are  drawn  from  the  experiments  of  Messrs.  Hodg- 
kinson  and  Fairbairn,  on  hot  and  cold  blast  iron,  as  published  in 
their  Reports  to  the  British  Association  in  1837. 

Table  exhibiting  the  results  of  experiments  by  Mr.  Hodgkinson 
on  bars  of  hot  blast  iron  5  feet  long,  with  a  rectangular  sec- 
tional area ;  the  bars  resting  horizontally  on  props  4  feet  6 
inches  apart ;  the  weight  being  applied  at  the  middle  of  the 
bar. 


Experiment  1. 

Experiment  13. 

Experiment  14. 

Rectangular  bar, 

1.00  inch  broad, 

1.00    "     deep. 

Weight  of  bar,  15  lbs.  2  oz. 

Rectangular  bar, 
1.03  inches  broad, 
3.00       "      deep. 

Rectangular  bar, 

1.02  inches  broad, 

4.98       "      deep. 

Weight  78  lbs. 

Weight  In 

lbs. 

= 
e  .,- 

•5  2 

II 

<* 

5*1 
Hi 

e 

--  ~ 
> 

i 

I 

M 

M 

i 

»  — 

3  S 

1 

I 
■i 

G 
C 

1 

16 

.037 

visible 

1474 

- 

.001 

5867 

.127 

- 

23 

.052 

increased 

1605 

.130 

.003 

6798 

.153 

.01 

30 

.070 

.001? 

1866 

.156 

.006 

7730 

.177 

- 

56 

.132 

.002 

2126   |    .185 

.010 

8661 

.207 

- 

112 

.271 

.008 

2388       .212 

.012 

9593 

.235 

- 

2£>4 

.588 

.037 

2649       .243 

.017 

10524 

.275 

.03 

I    336 

.940 

.087 

2910       .272 

.022 

11387 

broke 

- 

■    448 

1.360 

.181 

3172   !    .307 

.030 

- 

- 

- 

469 

broke 

- 

3433   1    .340 

.038 

- 

- 

- 

_ 

_ 

- 

3694   j    .378 

.050 

- 

- 

- 

- 

- 

- 

3956  :  broke 

- 

- 

- 

- 

Ultimate  deflection 
1.444  inches. 

Ultimate  deflection 
.416  inch. 

Ultimate  deflection 
.299  inch. 

86 


BUILDING  MATERIALS. 


Results  of  experiments,  by  the  same,  on  the  transverse  strength  of 
cold  blast  iron ;  length  of  bars,  and  distance  between  the  points 
of  support  the  same  as  in  the  preceding  Table. 


Experiment  1 

Experiment  12. 

Experiment  13. 

Rectangular  bar, 
1.025  inch  deep, 
1.002    "     broad. 
Weight,  15  lbs.  6  oz. 

Rectangular  bar, 

3.00  inches  deep, 

1.02      "       broad. 

Weight,  46  lbs.  8  oz. 

Rectangular  bar, 

4.98  inches  deep, 

1.03      "       broad 

Weight,  78  lbs. 

Weight  In 
lbs. 

a 

si 

02  j 

BfJB 

Deflection  In 
inches. 



Set  in 
inches. 

c 

MJS 

r 

.S 

"g-« 
ll 
S 

Set  in 
inches. 

16 

.033 

visible 

1082 

.091 

.003 

4936 

.110 

.013 

30 

.062 

increased 

1343 

.111 

.006 

5867 

.130 

- 

56 

.120 

.002 

1605        .138 

.008 

6798 

.153 

.020 

112 

.240 

.007 

1886 

.164 

.010 

7730 

.179 

.025 

168 

.370 

.014 

2126 

.190 

.012 

8662 

.195 

- 

224 

.510 

.028 

2388 

.220 

.015 

9593 

.219 

.034 

280 

.649 

.041 

2649 

.250 

.019 

10525 

.250 

.042 

336 

.798 

.061 

2910 

.281 

.026 

10588 

broke 

- 

392 

.953 

.084 

3172 

.310 

.031 

- 

- 

- 

448 

1.120 

.120 

3433 

.345 

.037 

- 

- 

- 

504 

1.310 

.170 

3694 

.378    j    .046 

- 

- 

- 

514 

it  bore 

- 

3825 

broke  1      - 

- 

- 

- 

518 

broke 

- 

- 

- 

- 

- 

- 

Ultimate  deflection 
1.36  inch. 

Ultimate  deflection 
0.395  inch. 

Ultimate  deflection 
0.252. 

312.  The  following  remarks  are  extracted  from  the  same  Re- 
port :  "I  had  remarked,  in  some  of  the  experiments,  that  the 
elasticity  of  the  bars  was  injured  much  earlier  than  is  generally 
conceived ;  and  that  instead  of  its  remaining  perfect  till  one  third, 
or  upwards,  of  the  breaking  weight  was  laid  on,  as  is  generally 
admitted  by  writers,  it  was  evident  that  jth,  or  less,  produced  in 
some  cases  a  considerable  set  or  defect  ol  elasticity  ;  and  judging 
from  its  slow  increase  afterwards,  I  was  persuaded  that  it  had  not 
come  on  by  a  sudden  change,  but  had  existed,  though  in  a  less 
degree,  from  a  very  early  period." 

"  From  what  has  been  stated  above,  deduced  from  experiments 
made  with  great  care,  it  is  evident  that  the  maxim  of  loading 
bodies  within  the  elastic  limit,  has  no  foundation  in  nature  ;  but 
it  will  be  considered  as  a  compensating  fact,  that  materials  will 
bear  for  an  indefinite  period  a  much  greater  load  than  has  hitherto 
oeen  conceived." 

313.  "We  may  admit,"  from  the  mean  results,  "tint  the 
strength  of  rectangular  bars  is  as  the  square  of  the  depth." 


STRENGTH  OF  MATERIALS. 


87 


314.  Effects  of  time  upon  the  deflections  caused  by,  a  perma- 
nent load,  on  the  middle  of  horizontal  bars. 

The  following  Table  exhibits  the  results  of  Mr.  Fairbairn's  ex- 
periments on  this  point.  The  experiments  were  made  on  bars 
5  feet  long,  1 .05  inch  deep  ;  the  one  of  cold  blast  iron,  1 .03  inch 
oroad ;  the  other  of  hot  blast,  1.01  broad;  distance  between  the 
pumts  of  support  4  feet  6  inches.  The  constant  weight  sus- 
pended at  the  centre  or  the  bars  was  280  lbs.  This  weight  re 
mained  on  from  March  11th,  1837,  to  June  23d,  1838. 


Cold  blast  iron. 
Deflection  in        Date  of  observation, 
inches. 

Temp. 

Hot  blast  iron. 
Deflection  in 
inches. 

Ratio  of  increase  of 
deflections. 

.930        1    March  11th,  1837, 
.963        j    June  23d,  1838, 

78° 

1.064 
1.107 

— 

.033        i    Increase, 

- 

.043 

1000  :  1303 

315.  Mr.  Fairbairn  in  his  Report  remarks  on  the  above  and 
like  results  :  "  The  hot  blast  bar  in  these  experiments  being  more 
deflected  than  the  cold  blast,  indicates  that  the  particles  are  more 
extended  and  compressed  in  the  former  iron,  with  the  same 
weight,  than  in  the  latter.  This  excess  of  deflection  may  in  some 
degree  account  for  the  rapidity  of  increase,  which  it  will  be  observed 
is  considerably  greater  in  the  hot  than  in  the  cold  blast  bar." 

"  It  appears  from  the  present  state  of  the  bars,  (which  indicate 
a  slow  but  progressive  increase  in  the  deflections,)  that  we  must 
at  some  period  arrive  at  a  point  beyond  their  bearing  powers  ;  or 
otherwise  to  that  position  which  indicates  a  correct  adjustment 
of  the  particles  in  equilibrium  with  the  load.  Which  of  the  two 
points  we  have  in  this  instance  attained  is  difficult  to  determine  : 
sufficient  data,  however,  are  adduced  to  show  that  the  weights 
are  considerably  beyond  the  elastic  limit,  and  that  cast  iron  will 
support  loads  to  an  extent  beyond  what  has  usually  been  consid- 
ered safe,  or  beyond  that  point  where  a  permanent  set  takes  place." 

316.  Effects  of  Temperature.  Mr.  Fairbairn  remarks  :  "  The 
infusion  of  heat  into  a  metallic  substance  may  render  it  more 
ductile,  and  probably  less  rigid  in  its  nature ;  and  I  apprehend  it 
will  be  found  weaker,  and  less  secure  under  the  effects  of  heavy 
strain.  This  is  observable  to  a  considerable  extent  in  the  experi- 
ments" on  transverse  strength  "ranging  from  26°  up  to  190°  Fahr." 

'  The  cold  blast  at  26°  and  190°,  is  in  strength  as  874  :  743, 
The  hot  blast  at  26°  and  190",  is  in  strength  as  811  :  731, 
oei'iig  a  diminution  ir  strength  as  100  :  85  for  the  cold  blast,  and 
100  to  90  for  the  hot  blast,  or  15  per  cent,  loss  of  strength  in  the 
cold  blast,  and  10  per  cent,  in  the  hot  blast." 

"  A  number  of  the  experiments  made  on  No.  3  iron  have  giver 


88 


BUILDING  MATERIALS. 


extraordinary  and  not  unfrequently  unexpected  results.     Gener 
ally  speaking,  it  is  an  iron  of  an  irregular  character,  and  presents 
less  uniformity  in  its  texture  than  either  the  first  or  second  quali- 
ties ;  in  other  respects  it  is  more  retentive,  and  is  often  used  for 
giving  strength  and  tenacity  to  the  finer  metals." 

"  At  212°  we  have  in  the  No.  3  a  much  greater  weight  sus- 
tained than  what  is  indicated  by  the  No.  2  at  190°  ;  and  at  600° 
there  appears  in  both  hot  and  cold  blast  the  anomaly  of  increased 
strength  as  the  temperature  is  advanced  from  boiling  water  to 
melted  lead,  arising  from  the  greater  strength  of  the  No.  3  iron.* 

317.  Influence  of  Form  in  Cast  Iron  upon  the  Transverse 
Strength  of  Beams.  Upon  no  point,  respecting  the  strength  of 
cast  iron,  have  the  experiments  of  Mr.  Hodgkinson  led  to  more 
valuable  results  to  the  engineer  and  architect,  than  upon  the  one 
under  this  head.  The  following  Tables  give  the  results  of  experi- 
ments on  bars  of  a  uniform  cross  section,  (thus  T",)  cast  from  hot 
and  cold  blast  iron.  The  bars  were  7  feet  long,  and  placed,  for 
breaking,  on  supports  6  feet  6  inches  asunder. 

Table  exhibiting  the  results  of  experiments  on  bars  of  hot  blast 
iron  of  the  form  of  cross  section  as  above. 


EXPERIMENT 

4. 

Experiment  5. 

Bar  broken 

with  the  rib  dow 

as  shown 

Bar  broken                  as  shown 

nward. 

with  the  rib  upward. 

a 

1 

C  £ 

|J 

Is 

i 

s 

f| 

|J 
gj 

1' 

7 

.015 

visible 

7 

—        not  visible 

14 

.032 

.001 

14 

.025 

visible 

21 

.046 

.002 

21 

.045 

.002 

28 

.064 

.004 

28 

.065 

.003 

56 

.130 

.005 

56 

.134 

.005 

112 

.273 

.020 

112 

.270 

.015 

168 

.444 

.035 

224 

.580 

.058 

224 

.618 

.058 

336 

.895 

.101 

280 

.813 

.093 

448 

1.224 

.155 

336 

1.030 

.130 

560 

1.585         .235 

364 

broke 

- 

672 

1.985         .330 

- 

- 

- 

784 

2.410 

.490 

- 

- 

- 

896 

3.450 

.722 

- 

- 

- 

1008 

4.140 

1.040 

- 

- 

- 

1064 

- 

_ 

- 

- 

- 

1120 

broke 

- 

Ultimate  deflection  1.138  inches. 

Fracture  c 
inches  1< 
this  form 
out. 

Ultima 

aused  by  a  wedge  2.92 
>ng  and  1.05  deep,  of 

te  deflection  4.830. 

STRENGTH  OF  MATERIALS. 


85 


Note.  The  annexed  diagram  shows  the 
form  of  the  uniform  cross  section  of  the 
bars.  The  linear  dimensions  of  the  cross 
section  in  the  two  experiments  were  as  fol- 
lows : 


Length  of  parallelogram  A  B  5  inches! 

Depth  "  AB  0.30 

Total  depth  of  bar       .     CD  1.55 

Breadth  of  rib  .     .     .     DE  0.36  "  J 


VExpt. 


4. 


5   inches'] 
0.30    "    I  v 
1.56    "    (Ex^- 
0.365  "  J 


Table  exhibiting  results  of  experiments  on  bars  of  cold  blast  iron 
5  feet  long,  of  the  same  form  of  cross  section  as  in  preceding 
Table. 


Experiment  4. 

Experiment  5. 

Bar  broken  ^^^^     with  rib 

Bar  broken                    with  rib 

downward. 

upward. 

Si 

r 

o  s 
fl 
|l 

■ 
to 

.5 

2  a? 

a" 

o  S 

11 

112 

.03 

— 

112 

.03 

_ 

224 

.07 

- 

224 

.07 

_ 

336 

.11 

- 

336 

.11 

_ 

392 

.13 

.005 

448 

.15 

- 

420 

.14 

.007 

560 

.19 

.005 

448 

.15 

.010 

616 

.21 

.010 

560 

.19 

.012 

672 

.23 

- 

672 

.23 

.015 

728 

- 

.015 

784 

.28 

.023 

784 

.27 

_ 

896 

.33 

.030 

896 

.31 

- 

952 

.35 

- 

1008 

•  .35 

_ 

980 

broke 

- 

1120 

.39 

_ 

- 

- 

- 

1344 

.48 

- 

4 

- 

- 

1568 

.57 

- 

- 

- 

- 

1792 

.67 

- 

- 

- 

- 

2016 

.80 

- 

- 

- 

- 

2240 

.95 

- 

— 

- 

- 

2296 

it  bore 

- 

- 

- 

- 

2352 

broke 

- 

Ultimate  deflection  3& 

Ultimate  deflection  1.03. 

Fracture  by  a  wedge  breaking 
out  as  in  Experiment  5,  Hot 
Blast. 

Note.  The  linear  dimensions  of  the  cross  section  of  the  ban, 
in  the  above  Table,  were  nearly  the  same  as  those  in  the  prece- 

12 


90  BUILDING  MATERIALS. 

ding  Table,  with  the  exception  of  the  total  depth  CD,  whk  h,  in 
these  last  two  experiments,  was  2.27  inches,  or  a  little  mors. 

318.  The  object  had  in  view  by  Mr.  Hodgkinson,  in  the  ex- 
periments recorded  in  the  two  preceding  Tables,  was  twofold ; 
the  one  to  ascertain  the  circumstances  under  which  a  permanent 
set,  or  injury  to  elasticity  takes  place  ;  the  other  to  ascertain  the 
effect  of  the  form  of  cross  section  on  the  transverse  strength  of 
cast  iron.  The  following  extracts  from  the  Report,  give  the 
principal  deductions  of  Mr.  Hodgkinson  on  these  points. 

"In  experiments  4  and  5,"  (on  hot  blast  iron,)  "which  were 
on  longer  bars  than  the  others,  cast  for  this  purpose,  and  for  an- 
other mentioned  further  on,  the  elasticity  (in  Expt.  4)  was  sensi- 
bly injured  with  7  lbs.,  and  in  the  latter  (Expt.  5)  with  14  lbs., 
the  breaking  weights  being  364  lbs.,  and  1120  lbs.  In  the  for- 
mer of  these  cases  a  set  was  visible  with  ^,  and  in  the  other 
with  r\  of  the  breaking  weight,  showing  that  there  is  no  weight, 
however  small,  that  will  not  injure  the  elasticity." 

"  When  a  body  is  subjected  to  a  transverse  strain,  some  of  its 
particles  are  extended  and  others  compressed  ;  I  was  desirous  to 
ascertain  whether  the  above  defect  in  elasticity  arose  from  ten- 
sion or  compression,  or  both.  Experiments  4  and  5  show  this  ;  in 
these  a  section  of  the  casting,  which  was  uniform  throughout,  had 

c 
the  form  J..     During  the  experiments  the  broad  part  ab  was  laid 

a      b 

horizontally  upon  supports  ;  the  vertical  rib  c  in  the  latter  experi- 
ment being  upward,  in  the  former  downward.  When  it  was 
downward  the  rib  was  extended,  when  upward  the  rib  was  com- 
pressed. In  both  cases  the  part  ab  was  the  fulcrum  ;  it  was  thin, 
and  therefore  easily  flexible  ;  but  its  breadth  was  such  that  it  was 
nearly  inextensible  and  incompressible,  comparatively,  with  the 
vertical  rib.  We  ma'y  therefore  assume,  that  nearly  the  whole 
flexure  which  takes  place  in  a  bar  of  this  form,  arises  from  the 
extension  or  compression  of  the  rib,  according  as  it  is  downward 
or  upward.  In  Expt.  4  we  have  extension  nearly  without  com- 
pression, and  in  Expt.  5  compression  almost  without  extension. 
These  experiments  were  made  with  great  care.  They  show  that 
there  is  but  little  difference  in  the  quantity  of  set,  whether  it 
arises  from  tension  or  compression." 

"  The  set  from  compression,  however,  is  usually  less  than  that 
from  extension,  as  is  seen  in  the  commencement  of  the  two  ex 
periments,  and  near  the  time  of  fracture  in  that  submitted  to  ten 
sion.  The  deflections  from  equal  weights  are  nearly  the  same 
whether  the  rib  be  extended  or  compressed,  but  the  ultimate 
strengths,  as  appears  from  above,  are  widely  different." 

319.  Form  of  Cast  Iron  Beam  best  adapted  to  resist  a  7  Van  a 


STREJ  GTH  OF  MATERIALS. 


91 


verse  Strain.  The  experiments  of  Mr.  Hodgkinson  on  this  sub- 
ject, published  in  the  Memoirs  of  the  Literary  and  Philosophical 
Society  of  Manchester,  Second  Series,  vol.  5,  arc  of  equal  in- 
terest with  those  just  detailed,  both  in  their  general  results  and 
practical  bearing.  From  these  experiments,  the  conclusion  diawr, 
is  that  the  form  of  beam  in  the  annexed  diagrams  is  the  most  fa* 
vorable  for  resistance  to  transverse  strains. 

Fig.  a. 


Fig.  b. 
e     t    f 


Flff.    Cm 

Fig.  a  represents  the  plan,  Fig.  b 
the  elevation,  and  Fig.  c  the  cross 
section  (enlarged)  at  the  middle  of 
the  beam.  From  the  Figs,  it  will 
be  seen,  that  the  beam  consists  of 
three  parts  ;  a  bottom  flanch  of  uni- 
form depth,  but  variable  breadth,  ta- 
pering from  the  centre  towards  the 
extremities,  where  the  points  of  sup- 
port would  be  placed,  so  as  to  form 
a  portion  of  the  common  parabola  on  each  side  of  the  axis  of  the 
beam,  the  vertex  of  each  parabola  being  at  the  centre  of  the  beam. 
The  object  of  this  form  of  flanch  was  to  make  it,  according  to 
theory,  the  strongest,  with  the  same  amount  of  material,  to  bear 
a  weight  uniformly  distributed  over  it.  The  top  flanch  is  of  a 
like  form,  but  of  much  smaller  breadth  and  depth  than  the  bottom 
one.  The  two  are  united  by  a  vertical  rib  of  uniform  depth  and 
breadth. 

The  following  are  the  relative  dimensions  of  this  form  of  beam 
which,  from  experiment,  gave  the  most  favorable  result. 


Distance  of  supports  .... 

.     4  ft. 

6    inches. 

Total  depth  of  beam   .... 

0  " 

5\       " 

Breadth  of  top  flanch  at  centre  of  beam 

2.33    " 

'•           bottom  flanch             " 

6.66    " 

Uniform  depth  of  top  flanch 

0.31    " 

"              bottom  flanch  . 

0.66   " 

Thickness  of  vertical  rib     . 

0.266  «' 

Total  area  of  cross  section 

6.4  square  incu 

Weight  of  beam         .... 

71  lbs 

92  BUILDING  MATERIALS 

"  This  beam  broke  in  the  middle  by  compression  with  26084 
lbs.,  or  11  tons  13  cwt.,  a  wedge  separating  from  its  upper 
side." 

"  The  weights  were  laid  gradually  and  slowly  on,  and  the  beam 
had  borne  within  a  little  of  its  breaking  weight  a  considerable  time, 
perhaps  half  an  hour." 

"  The  form  of  the  fracture  and  wedge  is  represented  in  the 
Fig.  b,  where  enf  is  the  wedge,  ef  =  5.1  inches,  tn  =  3.9  inches, 
angle  en/ =82°." 

"  It  is  extremely  probable,  from  this  fracture,  that  the  neutral 
point  was  at  n,  .he  vertex  of  the  wedge,  and  therefore  at  fths  the 
depth  of  the  beam,  since  3.9  =  £  x  5f  nearly." 

The  relative  dimensions  above  given  were  arrived  at  by  "  con- 
stantly making  small  additions"  to  the  bottom  flancli,  until  a  point 
was  reached  where  resistance  to  compression  could  no  longer  be 
sustained.  The  beams  of  this  form,  in  all  previous  experiments, 
having  yielded  by  the  bottom  flanch  tearing  asunder. 

"  The  great  strength  of  this  form  of  cross  section  is  an  indis- 
putable refutation  of -that  theory  which  would  make  the  top  and 
bottom  ribs  of  a  cast  iron  beam  equal." 

"  The  form  of  cross  section"  (as  above)  "  is  the  best  which  we 
have  arrived  at  for  the  beam  to  bear  an  ultimate  strain.  If  we 
adopt  the  form  of  beam,  (as  above,)  I  think  we  may  confidently 
expect  to  obtain  the  same  strength  with  a  saving  of  upwards  of 
ith  of  the  metal." 

320.  Rules  for  determining  the  ultimate  Strength  of  Cast 
Iron  Beams  of  the  above  forms.  From  the  results  of  his  experi- 
ments, Mr.  Hodgkinson  has  deduced  the  following  very  simple 
formulae,  for  determining  the  breaking  weight,  in  tons,  when  ap- 
plied at  the  middle  of  a  beam. 

Call  the  breaking  weight  in  tons,  W. 

Call  the  area  of  the  cross  section  of  the  bottom  flanch,  taken 
at  the  middle  of  the  beam,  a. 

Call  the  deptli  of  the  beam  at  the  middle  point,  d. 
Call  the  distance  between  the  supports,  I. 

Then 

ad 

W=26y, 

when  the  beam  has  been  cast  with  the  bottom  flanch  upward 
and 

when  the  beam  has  been  cast  on  its  side. 

321.  Effect  of  Hoiizontal  Impact  upon  cast  iron  bars,  and 


STRENGTH  OF  MATERIALS. 


93 


Measure  of  the  Resistance  offered  by  cast  iron  to  this  force.  The 
following  Tables  of  experiments  on  this  subject,  and  the  results 
drawn  from  them,  are  taken  from  a  paper  by  Mr.  Hodgkinson, 
published  in  the  Fifth  Report  of  the  British  Association. 

The  bars  under  experiment  were  impinged  upon  by  a  weight 
suspended  freely  in  such  a  position,  that  hanging  vertically  it  was 
in  contact  with  the  side  of  the  bar.  The  blow  was  given  by  al- 
lowing the  weight  to  swing  through  different  arcs.  The  bars 
were  so  confined  against  lateral  supports,  that  they  could  take  nc 
vertical  motion. 

Table  of  experiments  on  a  cast  iron  bar,  4  ft.  6  in.  long,  1  in. 
broad,  \  in.  thick,  weighing  1\  lbs.,  placed  with  the  broadside 
against  lateral  supports  4  ft.  asunder,  and  impinged  upon  by 
cast  iron  and  lead  balls  weighing  8£  lbs.,  swinging  through 
arcs  of  the  radius  12  feet. 


Impact  with  leaden  ball. 

Impact  with  iron  ball. 

OJ3 

"5  — 

u   = 

£•* 

"OS 

ii  - 

a  g 

-I 

■5<~  . 
u  o  ••. 

■  "■< 

1J 

*  s 
o 

u  o  oi 

-§5 

C  4> 

>.   y   O 

S2-S 

Pi 

«  5  u 

■3   o,^ 

l«5 

ItU 

£<o  = 

i  -.5 

1I.S 

l°-S 

111 

5 

o 

3 

o 

o 

O 

1 

6.5 

.24 

1 

6.5 

.23 

2 

13 

.46 

2 

14 

.46 

3 

19 

.73 

3 

20 

.65 

4 

27 

.97 

4 

29 

.98 

5 

34 

1.30 

5 

37 

1.32 

6 

47 

1.60 

6 

48 

1.65 

"  Before  the  experiments  on  impact  were  made  upon  this  bar, 
it  was  laid  on  two  horizontal  supports  4  feet  asunder,  and  weights 
gently  laid  on  the  middle  bent  it  (in  the  same  direction  that  it  was 
afterwards  bent  by  impact)  as  below  : 


28  lbs.  bent  it  .37  inch. 
56  lbs.      "      .77    " 


Elasticity  a  little  injured." 


94 


BUILDING  MATERIALS. 


Table  of  eoperiments  on  a  cast  iron  bar  7  ft.  long,  1 .08  in.  broad 
and  1.05  in.  thick,  weighing  23£  lbs.,  placed,  as  in  preceding 
experiments,  against  supports  6  ft.  6  in.  asunder,  and  bent  by 
impacts  in  the  middle.  Impinging  ball  of  cast  iron  weighing 
20|  los,     Radius  of  arcs  16  feet. 


Impact  upon  bar. 

Impact  upon  the 
weight. 

a 
*S£ 

•£ 

—  c 

C  - 

g<* 

o    . 

O  » 

—  <d 
of 

o- 

H 

IJ 

11 

11 

o- 

2 
3 
4 
5 

6 

7 
8 

.46 
.62 
.87 
1.03 
1.24 
1.44 
1.80 

2 
3 

4 
5 

6 
7 
8 
9 

.31 
.43 

.69 
.81 
1.04 
1.28 
1.41 
1.63 

The  results  in  the  3d  and  4th  columns  of  the  above  table  were 
derived  from  allowing  the  ball  to  impinge  against  a  weight  of  56 
lbs.,  hung  so  as  to  be  in  contact  with  the  bar. 

"  Before  the  experiments  on  impact,  the  beam  was  laid  on  two 
supports  6  ft.  6  in.  asunder,  and  was  bent  .78  in.  by  123  lbs., 
(including  the  pressure  from  its  own  weight,)  applied  gently  in 
the  middle." 

Tables  of  experiments  on  two  cast  iron  bars,  4  ft.  6  in.  long,  full 
inch  square,  weighing  14  lbs.  10  oz.  nearly,  placed  against 
supports  A  feet  apart,  and  impinged  upon  by  a  cast  iron  ball 
weighing  44  lbs.    Radius  16  ft. 


Impact  in  the  middle. 

Impact  at  one  fourth  the  length  from  the  middle 
of  the  bars. 

Chords  of  arcs  in 
feet. 

Mean  deflections 
of  the  two  bars 
in  inches. 

Chords  of  arcs 
in  feet. 

Mean    deflections 
of  the  two  bars 
in  inches. 

Mean  ratio  of  the 
deflections     in 
the  two  cases. 

2 

3 

4 

5 

5.5 

6 

.35 
.55 

.77 

.95 

1.05 

Broke  in  the 

middle 

2 

3 

4 

5 

5.5 

6 

.24 
.42 
.52 
.64 
.70 
Broke  at  the 
point  of  impact 

694 

The  results  in  the  1  st  of  the  above  Tables  are  from  bars  struck 


STRENGTH  OF  MATERIALS.  95 

in  the  middle,  those  in  the  2d  Table  are  from  bars  struck  at  the 
middle  point  between  the  centre  and  extremity  of  the  bar. 

322.  From  the  above  and  other  experiments  the  conclusion  is 
drawn,  "  that  a  uniform  beam  will  bear  the  same  blow,  whether 
struck  in  the  middle  or  half  way  between  that  and  one  end." 

"  From  all  the  experiments  it  appears,  that  the  deflection  is 
nearly  as  the  chord  of  the  arc  fallen  through,  or  as  the  velocity 
of  impact." 

The  following  conclusions  are  drawn  from  the  experiments. 

( 1 .)  "  If  different  bodies  of  equal  weight,  but  differing  consider- 
ably in  hardness  and  elastic  force,  be  made  to  strike  horizontally 
against  the  middle  of  a  heavy  beam  supported  at  its  ends,  all  the 
bodies  will  recoil  with  velocities  equal  to  one  another." 

(2.)  "  If,  as  before,  a  beam  supported  at  its  ends  be  struck 
horizontally  by  bodies  of  the  same  weight,  but  different  hardness 
and  elastic  force,  the  deflection  of  the  beam  will  be  the  same 
whichever  body  be  used." 

(3.)  "  The  quantity  of  recoil  in  a  body,  after  striking  against  a 
beam  as  above,  is  nearly  equal  to  (though  somewhat  below)  what 
would  arise  from  the  full  varying  pressure  of  a  perfectly  elastic 
beam,  as  it  recovered  its  form  after  deflection." 

Note.  This  last  conclusion  is  drawn  from  a  comparison  of  the 
results  of  experiment  with  those  obtained  from  calculation,  in 
which  the  beam  is  assumed  as  perfectly  elastic. 

(4.)  "  The  effect  of  bodies  of  different  natures  striking  against 
a  hard,  flexible  beam,  seems  to  be  independent  of  the  elasticities 
of  the  bodies,  and  may  be  calculated,  with  trifling  error,  on  a  sup- 
position that  they  are  inelastic." 

(5.)  "  The  power  of  a  uniform  beam  to  resist  a  blow  given 
horizontally,  is  the  same  in  whatever  part  it  is  struck." 

323.  From  the  results  of  the  experiments  of  Messrs.  Fairbairn 
and  Hodgkinson,  on  the  properties  of  cold  and  hot  blast  iron,  it  ap- 
pears that  the  ratio  of  their  resistances  to  impact  is  1000  to  1226.3, 
the  resistance  of  cold  blast  being  represented  by  1000;  the  re- 
sistance, or  power  of  the  beam  to  bear  a  horizontal  impact,  being 
measured  by  the  product  of  its  breaking  weight  from  a  transverse 
strain  at  the  middle  of  the  beam  and  its  ultimate  deflection.  This 
measure,  Mr.  Hodgkinson  remarks,  "  supposes  that  all  cast  iron 
bars  of  the  same  dimensions,  in  our  experiments,  are  of  the  same 
weight,  and  that  the  deflection  of  a  beam  up  to  the  breaking 
weight,  would  be  as  the  pressure.  Neither  of  these  is  true ; 
they  are  only  approximations  ;  but  the  difference  in  the  weights 
of  cast  iron  bars  of  equal  size  is  very  little,  and  taking  them  as 
the  same,  it  may  be  inferred  from  my  paper  on  Impact  upon 
Beams,  {Fifth  Report  of  the  British  Association,)  that  the  as- 
sumption above  gives  results  near  enough  for  practice." 


96 


BUILDING  MATERIALS. 


324.  Stbength  of  Wrought  Iron.  This  material,  from  its 
very  extensive  applications  in  structures  where  a  considerable 
tensile  force  is  to  be  resisted,  as  in  suspension  bridges,  iron  ties, 
&c,  has  been  the  subject  of  a  very  great  number  of  experiments. 
Among  the  many  may  be  cited  those  of  Telford  and  Brown  in 
England,  Duleau  in  France,  and  the  able  and  extensive  series 
upon  plate  iron  for  steam  boilers,  made  under  the  direction  of  the 
Franklin  Institute,  and  published  in  the  19th  and  20th  vols.  (New 
Series)  of  the  Journal  of  the  Institute. 

Resistance  to  Extension.  The  following  Tables  exhibit  the 
tensile  strength  of  this  material  under  ordinary  temperatures,  and 
in  the  different  states  in  which  it  is  used  for  structures. 

Table  exhibiting  the  Strength  of  Square  and  Round  bars  of 
Wrought  Iron. 


Length    of 

Extension  be- 

Breaking 

Tennle 

DISCBIPTIOS  07  IRON. 

piece*  in 

fore    rupture 

weight  in 

strength  per 

Author. 

feet, 

in  inches. 

tons. 

square  inch. 

Bar  1  inch  square,  Welsh 

1 

22.75 

29 

29 

Telford. 

"             "           Swedish 

1 

0.375 

29 

29 

« 

Round  oar,  2  in.  diam.  " 

1 

2.2 

100 

29.28      i 

" 

Bar,  1.31  inch  square    " 

3.5 

0.19 

40.95 

23.75 

Brown. 

'•    1.19 

3.5 

3.00 

33.50 

23.75 

" 

Bound  bar,  1.31  in.  diam.,  Russian 

3.5 

2.25 

36.10 

26.50 

" 

Bar,  1.25  inch  square,  Welsh    . 

3.5 

2.00 

38.05 

84.35 

" 

Round  bar,  2  in.  diam.     " 

12.5 

18.50 

82.75 

26.33 

" 

Bars  reduced  in  the  middle  by 

hammering  to  0.375  in.  square 

- 

- 

- 

31.35 

Brunei. 

0.50 

- 

- 

30.80      ! 

" 

Bar,         Missouri    . 

- 

- 

- 

21.38 

(  Franklin 
|  Institute. 

"    (slit  rods)    .... 

- 

- 

- 

22 .32 

" 

"            Tennessee 

- 

- 

- 

23.25      , 

" 

"            Salisbury,  Connecticut 

- 

- 

- 

25.89      I 

» 

"          >  Sieedisn 

- 

- 

- 

25.97 

« 

"         '  Centre  Co.,  Penn. 

- 

- 

- 

26.07      , 

" 

"            Lancaster  Co.,  Penn.  . 

- 

- 

- 

26.18     ; 

«• 

"    (cable  iron)                English 

- 

- 

- 

26.62      1 

" 

"        do.  hammer  hardened  " 

- 

- 

- 

31.70 

«• 

"                                            Russian 

- 

- 

- 

33.95 

" 

Wire,  0.333  in.  diam.  PhUlipsburg 

- 

- 

- 

37.58 

■ 

"    0:190    " 

- 

- 

— 

32.98 

" 

"     0.156      "                   " 

- 

- 

- 

39.80 

•« 

"      0.10        •'                  English 

~ 

~ 

m 

35.81 

Teiford. 

Table  exhibiting  the  Mean  Strength  of  Boiler  Iron,  per  square 
inch  in  lbs.,  cut  from  plates  with  shears. 


Process  of  manufacture. 

Rough  edge  bar. 

Edges  filed  uni- 
formly. 

Notches  filed  into 
bar  on  each  edge. 

Piled  iron  .... 
Hammered  plate 
Puddled  iron 

53,045 
47,506 
52,341 

56,081 
55,584 
51,039 

63,266 
58,447 
62,420 

It  is  remarked  in  the  Report  of  the  Sub-committee,  "  that  the 
inherent  irregularities  of  the  metal,  even  in  the  best  specimens, 


STRENGTH  OF  MATERIALS.  9? 

«vi  ether  of  rolled  or  hammered  iron,  seldom  fall  short  of  10  or  15 
per  cent,  of  the  mean  strength." 

J'rom  the  same  series  of  experiments,  it  appears  that  th 
i£rusgu    of  rolled  plate  lengthwise  is  about  6  per  cent,  greater 
than   rv  strength  crosswise. 

In  the  Tenth  Report  of  the  British  Association  in  1840,  Mr. 
Fairbairn  has  given  the  results  of  experiments  on  plate  iron  by 
Mr.  Hodgkinson,  from  which  it  appears  that  the  mean  strength 
A  iron  plates  lengthwise  is  22.52  tons. 
Crosswise  "   23.04    " 

Single-riveted  plates      "    18,590  lbs. 

Double-riveted  plates    "   22,258    " 

Representing  the  strength  of  the  plate  by  100. 

The  double-riveted  plates  will  be     .       .     70. 

The  single         "  "  56. 

325.  Professor  Barlow,  in  his  Report  to  the  Directors  of  the 
London  and  Birmingham  Railroad,  (Journal  of  Franklin  Insti- 
tute, July,  1835,)  states,  as  the  results  of  his  experiments,  that  a 
oar  of  malleable  iron  one  inch  square  is  elongated  the  ipooth  part 
of  its  length  by  a  strain  of  one  ton  ;  that  good  iron  is  elongated  the 
•oooth  part  by  a  strain  of  10  tons,  and  is  injured  by  this  strain, 
while  indifferent,  or  bad  iron  is  injured  by  a  strain  of  8  tons. 

From  the  Report  made  to  the  Franklin  Institute,  it  appears  that 
the  first  set,  or  permanent  elongation  may  take  place  under  very 
different  strains,  varying  with  the  character  of  the  material.  The 
most  ductile  iron  yields  permanently  to  a  low  degree  of  strain. 
The  extremes  by  which  a  permanent  set  is  given  vary  between 
the  0.416  and  0.872  of  the  ultimate  strength;  the  mean  of  thir- 
teen comparisons  being  0.641. 

326.  Resistance  to  Compression.  But  few  experiments  have 
been  published  on  the  resistance  of  this  material  to  compression. 
Rondelet  states  that  it  commences  to  yield  under  a  pressure  of 
about  70,800  lbs.  per  square  inch,  and  that  when  the  altitude  of 
the  specimen  tried  is  greater  than  three  times  the  diameter  of 
the  base  it  yields  by  bending.  Mr.  Hodgkinson  states  that  the 
circumstances  of  its  rupture  from  crushing  indicate  a  law  simi- 
lar to  what  obtains  in  cast  iron. 

327.  Resistance  to  a  Transverse  Strain.  The  following  Ta- 
bles exhibit  the  circumstances  of  deflection  from  a  transverse 
strain  on  bars  laid  on  horizontal  supports  ;  the  weight  being  ap- 
plied at  the  middle  of  the  bar. 

The  Table  I.  gives  the  results  on  bars  2  inches  square,  laid  on 
supports  33  inches  asunder;  Table  II.  the  results  on  bars  2 
inches  deep,  1 .9  in.  broad,  bearing  as  in  Table  I. 

13 


98 


BUILDING  MATERIALS. 


Table  I. 


Table  II. 


Weight  in  tons. 

Deflections    in 

inches  for  each 

half  ton. 

Weight  in  ton*. 

Deflections  is 

inches  for  eae'. 

half  ton 

.75 

.020 

.250 

_ 

1.00 

.020 

.50 

.016 

1.50 

.020 

1.00 

.022 

2.00 

.030 

1.50 

.020 

2.50 

.020 

2.00 

.026 

3.00 

Set 

2.25 

.018 

- 

- 

2.50 

.026 

- 

- 

2.75 

.038 

- 

- 

3.00 

.092 

The  above  experiments  were  made  by  Professor  Barlow,  an: 
published  in  his  Report  already  cited.  He  remarks  on  the  re- 
sults in  Table  II.,  that  the  elasticity  was  injured  by  2.50  I  i\s 
and  destroyed  by  3.00  tons. 

328.  Trials  were  made  to  ascertain  mechanically  the  position 
of  the  neutral  axis  on  the  cross  section.  Professor  Barlow  re- 
marks on  these  trials,  that  "  the  measurements  obtained  in  these 
experiments  being  tension  1.6,  compression  0.4,  giving  exactly 
the  ratio  of  1  to  4  in  rectangular  bars.  These  results  seem  the 
most  positive  of  any  hitherto  obtained ;  still  there  can  be  little 
doubt  this  ratio  varies  in  iron  of  different  qualities  ;  but  looking 
to  the  preceding  experiments,  it  is  probably  always  from  1  to  3, 
to  1  to  ft." 

329.  Effects  of  time  on  the  elongation  of  Wrought  Iron  from 
a  constant  strain  of  extension.  M.  Vicat  has  given,  in  the  An- 
nates de  Chimie  et  de  Physique,  vol.  54,  some  experiments  on 
this  point,  made  on  iron  wires  which  had  not  been  annealed,  by 
subjecting  four  wires,  respectively,  to  strains  amounting  to  the 
\,  the  £,  the  \,  and  f  of  their  tensile  strength,  during  a  period  of 
33  months. 

From  the  results  of  these  experiments  it  appears,  that  each 
wire,  immediately  upon  the  application  of  the  strain  to  which  it 
was  subjected,  received  a  certain  amount  of  extension. 

The  first  wire,  which  was  subjected  to  a  strain  of  \\h  its  ten- 
sile strength,  was  found  at  the  end  of  the  time  in  question  not  to 
have  acquired  any  increase  of  extension. 

The  second,  submitted  to  ^d  its  tensile  strength,  was  elongated 
0.027  in.  per  foot,  independently  of  the  elongation  it  at  first  re- 
ceived. 

The  third,  subjected  under  like  circumstances  to  a  strain  oi 
£th  its  tensile  strength,  was  elongated  0.40  in.  per  foot,  besidei 
its  first  elongation. 


STRENGTH  OF  MATERIALS.  99 

The  fourtn,  similarly  subjected  to  fths  the  tensile  strength,  was 
elongated  0.061,  hi  .sides  its  first  elongation. 

From  observations  made  during  the  experiments,  it  was  found 
'.hat,  reckoning  from  the  time  when  the  first  elongations  took  place, 
the  rapidity  of  the  subsequent  elongations  was  nearly  proportional 
to  the  times ;  and  that  the  elongations  from  strains  greater  than 
{tli  the  tensile  strength  are,  after  equal  times,  nearly  proportional 
to  the  strains. 

330.  M.  Vicat  remarks  ir  substance  upon  the  results  of  these 
experiments,  that  iron  wire,  when  not  annealed,  commenees  to 
exhibit  a  permanent  set  when  subjected  to  a  strain  between  the 
|  and  j  of  its  tensile  strength,  and  that  therefore  it  is  rendered 
probable  that  the  wire  ropes  of  a  suspension  bridge,  which  should 
be  subjected  to  a  like  strain,  would,  when  the  vibratory  motion  to 
which  such  structures  are  liable  is  considered,  yield  constantly 
from  year  to  year,  until  they  entirely  gave  way. 

M.  Vicat  farther  remarks,  in  substance,  that  the  measure  of  the 
resistance  offered  by  materials  to  strains  exerted  only  some  minutes, 
or  hours,  is  entirely  relative  to  the  duration  of  the  experiments. 
To  ascertain  the  absolute  measure  of  this  resistance,  which  should 
serve  as  a  guide  to  the  engineer,  the  materials  ought  to  be  sub- 
jected for  some  months  to  strains  ;  while  observations  should  be 
made  during  this  period,  with  accurate  instruments,  upon  the 
manner  in  which  they  yield  under  these  strains. 

331.  Effects  of  Temperature  on  the  Tensile  Strength  of 
Wrought  Iron  The  experiments  made  under  the  direction  of 
the  Franklin  Institute,  already  noticed,  have  developed  some  very 
curious  facts  of  an  anomalous  character,  with  respect  to  the  effect 
of  an  increase  of  temperature  upon  the  strength  of  wrought  iron. 
It  was  found  that  at  high  degrees  of  heat  the  tensile  strength  was 
greater  up  to  a  certain  point  than  was  exhibited  by  the  same  iron 
at  ordinary  temperatures.  The  Sub-committee  in  their  Report 
remark  :  "  This  circumstance  was  noted  at  212°,  392°,  and  572°, 
rising  by  steps  of  180°  each  from  32°,  at  which  last,  point  some 
trials  have  been  made  in  melting  ice.  At  the  highest  of  these 
points,  however,  it  was  perceived  that  some  specimens  of  the 
metal  exhibited  but  little,  if  any,  superiority  of  strength  over  that 
which  they  had  possessed  when  cold,  while  others  allowed  of 
being  heated  nearly  to  the  boiling  point  of  mercury,  before  they 
manifested  any  decided  indications  of  a  weakening  effect  from  in- 
crease of  temperature." 

"  It  hence  became  apparent  that  any  law,  taking  for  a  basis 
the  strength  of  iron  in  its  ordinary  condition,  and  at  common 
temperatures,  must  be  liable  to  great  uncertainty,  in  regard  to  its 
application  to  different  specimens  of  the  metal.  It  was  evident 
.hat  the  anomaly  above  referred  to  must  be  only  apparent,  and 


100 


BUIiDING  MATERIALS. 


that  the  tenacity  actually  exhibited  at  572°,  as  well  as  that  which 
prevails  while  the  iron  is  in  the  state  in  which  it  was  left  by 
forging,  or  rolling,  must  be  below  its  maximum  tenacity." 

From  the  experiments  made  upon  several  bars  of  the  same 
iron,  it  appeared  that  their  "maximum  tenacity  was  15  17  pel 
cent,  greater  than  their  mean  strength  when  tried  cold." 

Calculating  the  maximum  tenacity  in  other  experiments  from 
this  standard,  the  Sub-committee  have  drawn  up  the  following 
Table  exhibiting  the  relations  between  diminutions  from  the  max- 
imum tenacity  and  the  degrees  of  temperature  by  which  they  are 
caused,  from  which  the  curve  representing  the  law  of  these  rela- 
tions can  be  constructed. 


Table, 


No.  of  the  com- 

Observed tem- 

Observed tem- 

Observed dimi- 
nution of  te- 
nacity. 

Power  of  the  temperature 
which    represents    th*: 

parison. 

peratures. 

peratures — 80°. 

diminution  of  tenacity 
at  each  point. 

1 

520° 

440° 

.0738 

2.25 

2 

570 

490 

.0869 

2.17 

3 

596 

516 

.0899 

2.38 

4 

662 

582 

.1155 

2.67 

5 

770 

690 

.1627 

2.85 

6 

824 

744 

.2010 

2.94 

7 

932 

852 

.3324 

2.97 

8 

1030 

950 

.4478 

2.92 

9 

1111 

1031 

.5514 

2.63 

10 

1155 

1075 

.6000 

2.60 

11 

1237 

1157 

.6622 

2.41 

12 

i 

1317 

1237 

.7001 

2.14 
Mean  2.58 

The  Sub-committee  remark  on  ths  construction  of  the  above 
Table  "As  some  of  the  experiment  which  furnished  the  stand- 
ards of  comparison  for  strength  at  ordinary  temperatures,  were 
made  at  80°,  and  as  at  this  point  small  variations  with  respect  to 
heat  appear  to  affect  but  very  slightly  the  tenacity  of  iron,  it  was 
conceived  that  for  practical  purposes,  at  least,  the  calculations 
might  be  commenced  from  that  point." 

"  It  will  be  found  that  with  the  exception  of  a  slight  anomaly 
between  520°  and  570°,  amounting  to  —  .08,  the  numbers  express- 
_ng  the  ratios  between  the  elevations  of  temperature,  and  the 
diminutions  of  tenacity,  constantly  increase  until  we  reach  932°, 
at  which  it  is  2.97,  and  that  from  this  point  the  ratio  of  diminu- 
tion decreases  to  the  limits  of  our  range  of  trials,  1317°,  where  it 
is  2.14.  It  will  also  be  observed,  that  thf  diminution  of  tenacity 
at  932°,  where  the  law  changes  from  an  ir  ;reasing  to  a  decreasing 


STRENGTH  OF  MATERIALS. 


101 


rate  of  diminution,  is  almost  precisely  one  third  of  the  total,  or 
maximum  strength  of  the  iron  at  ordinary  temperatures." 

From  the  mean  of  all  the  rates  in  the  above  Table  the  follow- 
ing rule  is  deduced  :  "  the  thirteenth  power  of  the  temperature 
above  80°  u  proportionate  to  the  fifth  power  of  the  diminution 
frorr.  the  maximum  tenacity  P 

Professor  W.  R.  Johnson,  a  member  of  the  sub-committee, 
has  since  applied  the  results  developed  in  the  preceding  experi- 
ments to  practical  purposes,  in  increasing  the  tenacity  of  wrought 
iron  by  subjecting  it  to  tension  under  a  high  degree  of  tempera- 
ture, before  using  it  for  purposes  in  which  it  will  have  to  undergo 
considerable  strains,  as,  for  example,  in  chain  cables,  &c. 

This  subject  was  brought  by  Prof.  Johnson  before  the  Board 
of  Navy  Commissioners  in  1841 ;  subsequently,  experiments  were 
made  by  him  under  direction  of  the  Navy  Department,  the  results 
of  which,  as  exhibited  in  the  following  Table,  were  published  in 
the  Senate  Public  Documents,  (1)  28th  Congress,  2d  Session, 
p.  641.    Dec.  3,  1844. 

Table  of  the  effects  of  Thermo-tension  on  the   Tenacity  and 
Elongation  of  Wrought  Iron. 


KIND   OF   IRON. 

Strength 
of  cola. 

Strength   af- 
ter treating 
with  Ther- 
mo-tension. 

Gain  of 
length. 

Gain  of 

strength  by 
the  treat- 
ment. 

i 

Total  gain 
of  value. 

Tredegar,  No.  1,  round  iron 

Do.                     do. 
Tredegar,  square  bar  iron 
Tredegar,  No.  3,  round  iron 
Salisbury,  round,  (Ames") 

Mean, 

60 
60 
60 
58 
105.87 

71.4 

72.0 

67.2 

68.4 

121.0 

6.51 

6.51 

6.77 

5.263 

3.73 

19.00 
20.00 
12.00 
17.93 
14.29 

25.51 
26.51 

18.77 
23.19 
18.02 

— 

—      . 

5.75 

16.64 

22.40 

Prof.  Johnson  in  his  letter  remarks  :  "  It  will  be  observed  that 
in  these  experiments  the  temperature  has,  with  a  view  to  economy 
of  time,  been  limited  to  400°,  whereas  the  best  effects  of  the  pro- 
cess have  generally  been  obtained  heretofore  when  the  heat  has 
been  as  high  as  575°." 

332.  Resistance  of  Iron  Wire  to  Impact.  The  following  Ta- 
ble of  experiments  gives  the  results  obtainc :  by  Mr.  Hodgkinson, 
by  suspending  an  iron  ball  at  the  end  of  a  w_re,  (diameter  No.  17,) 
and  letting  another  iron  ball  impinge  upon  it  from  different  alti- 
tudes. The  suspended  and  impinging  balls  had  holes  drilled 
through  them,  through  which  the  wire  passed.  A  disc  of  lead 
was  placed  on  the  suspended  ball  to  receive  the  blow,  and  lesser 
the  recoil  from  elasticity.  • 


102 


BUILDING  MATERIALS. 


Table. 


- 
Length  tf 

Weight  of 
striking  ball. 

Weight  of 
suspended 

Ha  ght  fallen  through  by  striking 

Wire  broke 
with  ball  fall- 

1 
Remarks. 

ball  kihI  lead. 

ing  through. 

t                | 

ft.    in. 

lbs.  oz. 

lbs.  oz. 

25     0 

5    14 

0     9 

2,  24,  3,  34,  4, 
(repeated)  84,3,34,4,  44, 

?*« 

No  lead. 

24     0 

6      0 

10      1 

7, 

ai' 

■ 

— 

— 

— 

(repeated  with  fresh  wire.)  6, 

The  'lire  usually 

— 

— 

44     0 

1,  2,  3,  4,  5,  6,  64,  7, 

74 

broke  near  the  point  1 

— 

— 

80     8 

6,  64,  7,  7\,  8,  8  ,  9, 

94 

of  impact,  and   it  1 
.was  adjusted  to  its  j 
length,  if  fresh  wire 

— 

— 

80     0 

8,  8.,  9,  94,  10,  104, 

11 

— 

— 

125     0 

8,  8^,  9,  94,  10, 

104 

were  not  used  by  a  { 

— 

40      0 

10      1 

3,  4  inches, 

5  inches 

reserve  at  the  top. 

— 

— 

80     8 

2,  3,  4,  5,  6  inches, 

7     ds>. 

— 

— 

89     0 

4,  5  inches, 

6     do. 

Broke  one  inch 

24     8 

85      0 

44     0 

2  inches, 

3     do. 

from  top. 

The  following  observations  are  made  by  Mr.  Hodgkinson  : 
"  To  ascertain  the  strength  and  extensibility  of  this  wire,  it  was 
broken  in  a  very  careful  experiment  with  252^  lbs.,  suspended 
at  its  lower  end,  and  laid  gradually  on.  And  to  obtain  the  incre- 
ment of  a  portion  of  the  wire  (length  24  ft.  8  in.)  when  loaded  by 
a  certain  weight,  it  had  139  lbs.  hung  at  the  bottom,  and  when 
89  lbs.  were  taken  off  the  load,  the  wire  decreased  in  length  .39 
inch. 

"  Should  it  be  suggested  that  the  wire  by  being  frequently  im- 
pinged upon  would  perhaps  be  much  weakened,  the  author  would 
beg  to  refer  to  a  paper  of  his  on  Chain  Bridges,  Manchester  Me- 
moirs, 2d  series,  vol.  5,  where  it  is  shown  that  an  iron  wire  broken 
by  pressure  several  times  in  succession  is  very  little  weakened, 
and  will  nearly  bear  the  same  weight  as  at  first." 

"  The  first  of  the  preceding  experiments  on  wires  are  the  only 
ones  from  which  the  maximum  can,  with  any  approach  to  cer- 
tainty, be  inferred ;  and  we  see  from  them  that  the  wire  resisted 
the  impulsion  with  the  greatest  effect  when  it  was  loaded  at  bot- 
tom with  a  weight,  which,  added  to  that  of  the  striking  body,  was 
a  little  more  than  one  third  of  the  weight  that  would  break  the 
wire  by  pressure." 

"  From  these  experiments  generally,  it  appears  that  the  wire 
was  weak  to  bear  a  blow  when  lightly  loaded." 

"  These  last  experiments  and  remarks,  and  some  of  the  prece- 
ding ones,"  (on  horizontal  impact,)  "  show  clearly  the  benefit  of 
giving  considerable  weight  to  elast';  structures  subject  to  impact 
and  vibration." 

333.  Resistance  to  Torsion  of  Wrought  and  Cast  Iron.  The 
following  Table  exhibits  the  results  of  experiments  made  by 
Mr.  Dunlop,  at  Glasgow,  on  round  bars  of  wrought  iron.  The 
twisting  weights  were  applied  with  an  arm  of  lever  14  feet  2 
inches. 


STRENGTH  OF  MATERIALS. 


103 


Length  of  bars 

Diameter  of  bars  Weight  in  lbs.  pro- 

In  inches. 

in  inches. 

dueing  rupture. 

2? 

2 

250 

3 

2J 

384 

3 

2? 

408 

3 

700 

4 

3r 

1170 

5 

H 

1240 

5 

3? 

1662 

5 

4 

1938 

6 

4} 

2158 

Table  of  experiments  made  by  Mr.  G.  Rennie  upon  Cast  and 
Wrought  Iron.     Weight  applied  at  an  arm  of  lever  of  2  feet. 


i 

Length  of 

Size  of 

Mean  break- 

MATERIAL. 

blocks  in 

sectional 

ing  weight 

I 

inches. 

area. 

in  lbs. 

lbs.     oz. 

Iron  cast  horizontally         .... 

0 

9     15 

"         vertically 

0 

10     10 

"         horizontally 

1 

1 

7      3 

U                          U 

8       1 

K                  (i 

1 

8       8 

"         vertically 

i 

10       1 

u                 u 

3 

8       9 

u                           M 

1 

8       5 

H                             U 

6 

9     12 

"         horizontally 

0 

93     12 

((                               u 

0 

74 

; 

10 

52 

Wrought  iron,  (English) 

0 

10       2 

"               (Swedish) 

0 

9       8 

334.  Strength  of  Copper.  The  various  uses  to  which  cop- 
per is  applied  in  consl ructions,  render  a  knowledge  of  its  resist- 
ance under  various  circumstances  a  matter  of  great  interest  to  the 
engineer. 

Resistance  to  Extension.  The  resistance  of  cast  copper  on 
the  square  inch,  from  the  experiments  of  Mr.  G.  Rennie,  is  8.51 
tons,  that  of  wrought  copper  reduced  per  hammer  at  15.08  tons. 
Copper  wire  is  stated  to  bear  27.30  tons  on  the  square  inch. 
From  the  experiments  made  under  the  direction  of  the  Franklin 
Institute,  already  cited,  the  mean  strength  of  rolled  sheet  copper 
is  stated  at  14.35  tons  per  square  inch. 

Resistance  to  Compression.  Mr.  Rennie's  experiments  on 
cubes  of  one  fourth  of  an  inch  on  the  edge,  give  for  the  crushing 


104 


BUILDING  MATERIALS. 


weight  of     cube  of  cast  copper  7318  lbs.,  and  of  wrought  coppei 
6440  lbs. 

335.  Effects  of  Temperature  on  Tensile  Strength.  The  ex- 
periments already  cited  of  the  Franklin  Institute,  show  that  the 
difference  in  strength  at  the  lower  temperatures,  as  between  60° 
and  90°,  is  scarcely  greater  than  what  arises  from  irregularities 
in  the  structure  of  the  metal  at  ordinary  temperatures.  At  550° 
Fahr.  copper  loses  one  fourth  of  its  tenacity  at  ordinary  tempera- 
tures, at  817°  precisely  one  half,  and  at  1000°  two  thirds. 

Representing  the  results  of  experiments  by  a  curve  of  which 
the  ordinates  represent  the  temperatures  above  32°,  and  the  ab- 
scissas the  diminutions  of  tenacity  arising  from  increase  of  tern 
perature,  the  relations  between  the  two  will  be  thus  expressed  : 
the  squares  of  the  diminutions  are  as  the  cubes  of  the  tempera- 
tures. 

336.  Strength  of  other  Metals.  Mr.  Rennie  states  the 
tenacity  of  cast  tin  at  2.11  tons  per  square  inch  ;  and  the  resist- 
ance to  compression  of  a  small  cube  of  \  of  an  inch  on  an  edge 
at  966  lbs. 

In  the  same  experiments,  the  tenacity  of  cast  lead  is  stated  at 
0.81  tons  per  square  inch ;  and  the  resistance  of  a  small  cube  of 
same  size  as  in  preceding  paragraph  at  483  lbs. 

In  the  same  experiments,  the  tenacity  of  hard  gun-metal  is 
stated  at  16.23  tons  ;  that  of  fine  yellow  brass  at  8.01  tons.  The 
resistance  to  compression  of  a  cube  of  brass  the  same  as  before- 
mentioned,  is  stated  at  10304  lbs. 

337.  Linear  Dilatation  of  Metals  by  Heat.  The  following 
Table  is  taken  from  results  of  experiments  on  the  dilatation  of 
solids,  by  Professor  Daniell,  published  in  the  Philosophical 
Transactions,  1831. 

Table  of  Dimensions  which  a  bar  takes  whose  length  at  62°  is 

1.0000. 


At  212°,  (150°.) 

At  662°,  (600°.) 

At  point  of  fusion. 

Iron,  (wrought)    . 

1.000984 

1.004483 

1.018378* 

Iron,  (cast) 

1.000893 

1.003943 

1.016389 

Zinc 

1.002480 

1.008527 

1.012621 

Copper 

1.001430 

1.006347 

1.024376 

Lead 

1.002323 

— 

1.009072 

Tin      .    _     . 

1.001472 

— 

1.003798 

Brass,  (zinc  £) 

1.001787 

1.007207 

1.021841 

Bronze,  (tin  £) 

1.001541 

1.007053 

1.016336 

Pevrter,  (tin  \) 

1.001696 

— 

1.003776 

1 

*  At  fusing  point  of  cast  iron 


STRENGTH  OF  MATERIALS.  105 

338.  Adhesion  of  Iron  Spikes  to  Timber.  The  following 
Tables  and  results  are  taken  from  an  article,  by  Professor 
Walter  R.  Johnson,  published  in  the  Journal  of  the  Franklin 
Institute,  vol.  19,  1837,  giving  the  details  of  experiments  made 
by  him  on  spikes  of  various  forms  driven  into  different  kinds  of 
timber. 

339.  The  first  series  of  experiments  was  made  with  Burden's 
plain  square  spike,  the  flanched,  grooved,  and  swell  spike,  and 
the  grooved  and  swelled  spike.  The  timber  was  seasoned  Jersey 
yellow  pine,  and  seasoned  white  oak. 

From  these  experiments  it  results,  that  the  grooved  and  swelled 
form  is  about  5  per  cent,  less  advantageous  than  the  plain,  in  yel- 
low pine,  and  about  18£  per  cent,  superior  to  the  plain  in  oak. 
The  advantage  of  seasoned  oak  over  the  seasoned  pine,  for  re- 
taining plain  spikes,  is  as  1  to  1.9,  and  for  grooved  spikes  as  1  to 
2.37. 

340.  The  second  series  of  experiments,  in  which  the  timber 
was  soaked  in  water  after  the  spikes  were  driven,  gave  the  fol- 
lowing results. 

For  swelled  and  grooved  spikes,  the  order  of  retentiveness  was, 
1  locust ;  2  white  oak ;  3  hemlock ;  4  unseasoned  chesnut ;  5 
yellow  pine. 

For  grooved  spike  without  swell,  the  like  order  is — 1  unsea- 
soned chesnut ;  2  yellow  pine  ;  3  hemlock. 

The  swelled  and  grooved  spike  was,  in  all  cases,  found  to  be 
inferior  to  the  same  spike  with  the  swell  filed  off. 

341.  The  third  series  of  experiments  gave  the  following  results. 
Thoroughly  seasoned  oak  is  twice,  and  thoroughly  seasoned 

locust  2f  times  as  retentive  as  unseasoned  chesnut. 

The  forces  required  to  extract  spikes  are  more  nearly  propor 
tional  to  the  breadths  than  to  either  the  thickness  or  the  weights 
of  the  spikes.  And,  in  some  cases,  a  diminution  of  thickness 
with  the  same  breadth  of  spike  afforded  a  gain  in  retentiveness. 

"  In  the  softer  and  more  spongy  kinds  of  wood  the  fibres,  in- 
stead of  being  forced  back  longitudinally  and  condensed  upon 
themselves,  are,  by  driving  a  thick,  and  especially  a  rather  ob 
tusely-pointed  spike,  folded  in  masses  backward  and  downward  so 
as  to  leave,  in  certain  parts,  the  faces  of  the  grain  of  the  timber 
in  contact  with  the  surface  of  the  metal." 

"  Hence  it  appears  to  be  necessary,  in  order  to  obtain  the 
greatest  effect,  that  the  fibres  of  the  wood  should  press  the  faces 
as  nearly  as  possible  in  their  longitudinal  direction,  and  with  equal 
intensities  throughout  the  whole  length  of  the  spike." 

The  following  is  the  order  of  superiority  of  the  spikes  from 
that  of  the  ratio  of  their  weights  and  extracting  forces  respeo 
tively. 

14 


106  BUILDING  MATERIALS. 


1.  Narrow  flat    . 

7.049  ratio  of  weight 

to  exi 

Iracting  force 

2.  Wide  flat 

5.712 

M 

u 

3.  Grooved  but  not  swelled 

5.662 

.( 

M 

H 

4.  Grooved  and  not  notched 

5.300 

H 

«( 

H 

5.  Grooved  and  swelled 

4.624 

(( 

(( 

(« 

6.  Burden's  patent 

4.509 

H 

(( 

H 

7.  Square  hammered  . 

4.129 

M 

«( 

M 

8.  Plain  cylindrical     . 

3.200 

If 

(( 

M 

"  All  the  experiments  prove  that  when  a  spike  is  once  started, 
the  force  required  for  its  final  extraction  is  much  less  than  that 
which  produced  the  first  movement." 

"  When  a  bar  of  iron  is  spiked  upon  wood,  if  the  spike  be 
driven  until  the  bar  compresses  the  wood  to  a  great  degree,  the 
recoil  of  the  latter  may  become  so  great  as  to  start  back  the  spike 
for  a  short  distance  after  the  last  blow  has  been  given." 

342.  From  the  fourth  series  of  experiments  it  appears,  that  the 
spike  tapering  gradually  towards  the  cutting  edge,  gives  better 
results  than  those  with  more  obtuse  ends. 

That  beyond  a  certain  limit  the  ratio  of  the  weight  of  the  spike 
to  the  extracting  force  begins  to  diminish  ;  "  showing  that  it  would 
be  more  economical  to  increase  the  number  rather  than  the  length 
of  the  spikes  for  producing  a  given  effect." 

"  That  the  absolute  retaining  power  of  unseasoned  chesnut  on 
square  or  flat  spikes  of  from  two  to  four  inches  in  length,  is  a 
little  more  than  800  lbs.  for  every  squrre  inch  of  their  two  faces 
which  condense  longitudinally  the  fibres  of  the  timber." 


MASONRY.  107 


MASONRY. 

343.  Masonry  is  the  art  of  raising  structures,  in  stone,  brick, 
and  mortar. 

344.  Masonry  is  classified  either  from  the  nature  of  the  mate- 
.rial,  as  stone  masonry,  brick  masonry,  and  mixed,  or  that  which 

is  composed  of  stone  and  brick ;  or  from  the  manner  in  which 
the  material  is  prepared,  as  cut  stone  or  ashlar  masonry,  rubble 
stone  or  rough  masonry,  and  hammered  stone  masonry;  or, 
finally,  from  the  form  of  the  material,  as  regular  masonry,  and 
irregular  masonry. 

345.  Cut  Stone.  Masonry  of  cut  stone,  when  carefully  made, 
is  stronger  and  more  solid  than  that  of  any  other  class  ;  but,  owing 
to  the  labor  required  in  dressing,  or  preparing  the  stone,  it  is  also 
the  most  expensive.  It  is,  therefore,  mostly  restricted  to  those 
works  where  a  certain  architectural  effect  is  to  be  produced  by 
the  regularity  of  the  masses,  or  where  great  strength  is  indispen- 
sable. 

346.  Before  explaining  the  means  to  be  used  to  obtain  the 
greatest  strength  in  cut  stone,  it  will  be  necessary  to  give  a  few 
definitions  to  render  the  subject  clearer. 

In  a  wall  of  masonry,  the  term  face  is  usually  applied  to  the 
front  of  the  wall,  and  the  term  back  to  the  inside  ;  the  stone 
which  forms  the  front,  is  termed  the  facing;  that  of  the  back, 
the  backing ;  and  the  interior,  the  filling.  If  the  front,  or  back 
of  the  wall,  has  a  uniform  slope  from  the  top  to  the  bottom,  this 
slope  is  termed  the  batter,  or  batir. 

The  term  course  is  applied  to  each  horizontal  layer  of  stone 
in  the  wall :  if  the  stones  of  each  layer  are  of  equal  thickness 
throughout,  it  is  termed  regular  coursing ;  if  the  thicknesses  are 
unequal,  the  term  random,  or  irregular  coursing,  is  applied. 
The  divisions  between  the  stones,  in  the  courses,  are  termed  the 
joints ;  the  upper  surface  of  the  stones  of  each  course  is  also, 
sometimes,  termed  the  bed,  or  build. 

The  arrangement  of  the  different  stones  of  each  course,  or  of 
contiguous  courses,  is  termed  the  bond. 

347.  The  strength  of  a  mass  of  cut  stone  masonry  will  depend 
on  the  size  of  the  blocks  in  each  course  ;  on  the  accuracy  of  the 
dressing  ;  and  on  the  bond  used. 

348.  The  size  of  the  blocks  varies  with  the  kind  of  stone,  and 
the  nature  of  the  quarry.  From  some  quarries  the  stone  may  be 
obtained  of  any  required  dimensions  ;  others,  owing  to  some  pe- 
culiarity in  the  formation  of  the  stone,  only  furnish  blocks  of  smalJ 


1 08  MASONRY. 

size.  Again,  tie  strength  of  some  stones  is  so  great  as  to  admit 
of  their  being  used  in  blocks  of  any  size,  without  danger  to  the 
stability  of  the  structure,  arising  from  their  breaking ;  others  can 
only  be  used  with  safety,  when  the  length,  breadth,  and  thickness 
of  the  block  bear  certain  relations  to  each  other.  No  fixed  rule 
can  be  laid  down  on  this  point :  that  usually  followed  by  builders, 
is  to  make,  with  ordinary  stone,  the  breadth  at  least  equ»l  to  the 
thickness,  and  seldom  greater  than  twice  this  dimension,  and  to 
limit  the  length  to  within  three  times  the  thickness.  AYhen  the 
breadth  or  the  length  is  considerable,  in  comparison  with  the 
thickness,  there  is  danger  that  the  block  may  break,  if  any  un- 
equal settling,  or  unequal  pressure  should  take  place.  As  to  the 
absolute  dimensions,  the  thickness  is  generally  not  less  than  one 
foot,  nor  greater  than  two  ;  stones  of  this  thickness,  with  the  rel- 
ative dimensions  just  laid  down,  will  weigh  from  1000  to  8000 
pounds,  allowing,  on  an  average,  160  pounds  to  the  cubic  foot. 
With  these  dimensions,  therefore,  the  weight  of  each  block  will 
require  a  very  considerable  power,  both  of  machinery  and  men, 
to  se*t  it  on  its  bed. 

349.  For  the  coping  and  top  courses  of  a  wall,  the  same  ob- 
jections do  not  apply  to  excess  in  length  :  but  this  excess  may,  on 
the  contrary,  prove  favorable ;  because  the  number  of  top  joints 
being  thus  diminished,  the  mass  beneath  the  coping  will  be  better 
protected,  being  exposed  only  at  the  joints,  which  cannot  be  made 
water-tight,  owing  to  the  mortar  being  crushed  by  the  expansion 
of  the  blocks  in  warm  weather,  and,  when  they  contract,  being 
washed  out  by  the  rain. 

350.  The  closeness  with  which  the  blocks  fit  is  solely  depen- 
dent on  the  accuracy  with  which  the  surfaces  in  contact,  are 
wrought  or  dressed ;  if  this  part  of  the  work  is  done  in  a  slovenly 
manner,  the  mass  will  not  only  present  open  joints  from  any  in- 
equality in  the  settling ;  but,  from  the  courses  not  fitting  accurately 
on  their  beds,  the  blocks  will  be  liable  to  crack  from  the  unequal 
pressure  on  the  different  points  of  the  block. 

351.  The  surfaces  of  one  set  of  joints  should,  as  a  prime  con- 
dition, be  perpendicular  to  the  direction  of  the  pressure  :  by  this 
arrangement,  there  will  be  no  tendency  in  any  of  the  blocks  to 
slip.  In  a  vertical  wall,  for  example,  the  pressure  being  down- 
ward, the  surfaces  of  one  set  of  joints,  which  are  the  beds,  must 
be  horizontal.  The  surfaces*  of  the  other  set  must  be  perpen- 
dicular to  these,  and,  at  the  same  lime,  perpendicular  to  the  face, 
or  to  the  back  of  the  wall,  according  to  the  position  of  the  stones 
in  the  mass  ;  two  essential  points  will  thus  be  attained  ;  the  an- 
gles of  the  blocks,  at  the  top  and  bottom  of  the  course,  and  at  the 
face  or  back,  will  be  right  angles,  and  the  block  will  therefore  be 
as  strong  as  the  nature  of  the  stone  will  admit.     The  principles 


MASONRY.  109 

here  applied  to  a  vertical  wall,  are  applicable  in  all  cases*  what- 
ever may  be  the  direction  of  the  pressure  and  the  form  of  the  ex- 
terior surfaces,  whether  pane  or  curved. 

352.  A  modification  of  this  principle,  however,  may  in  some 
cases  be  requisite,  arising  from  the  strength  of  the  stone.  It  is 
laid  down  as  a  rule,  drawn  from  the  experience  of  builders,  that 
uo  stone  work  with  angles  less  than  60°  will  offer  sufficient 
strength  and  durability  to  resist  accidents,  and  the  effects  of  the 
weather.  If,  therefore,  the  batter  of  a  wall  should  be  greater  than 
60°,  which  is  about  7  perpendicular  to  4  base,  the  horizontal 
joints  (Fig.  6)  must  not  be  carried  out  in  the  same  plane,  to  the 

Fig.  6— Represents  the  arrangement  of  stone  with 
abutting,  or  elbow  joints  for  very  inclined  sur- 
faces. 

A,  lace  of  the  block, 
c,  elbow  joint. 

B,  buttress  block,  termed  a  newell  stone. 


face  or  back,  but  be  broken  off  at  right  angles  to  it,  so  as  to 
form  a  small  abutting  joint  of  about  4  inches  in  thickness.  As 
the  batter  of  walls  is  seldom  so  great  as  this,  except  in  some  cases 
of  sustaining  walls  for  the  side  slopes  of  earthen  embankments, 
this  modification  in  the  joints  will  not  often  occur ;  for,  in  a 
greater  batter,  it  will  generally  be  more  economical,  and  the 
construction  will  be  stronger,  to  place  the  stones  of  the  exterior  in 
offsets,  the  exterior  stone  of  one  course,  being  placed  within  the 
exterior  one  of  the  course  below  it,  so  as  to  give  the  required 
general  direction  of  the  batter.  The  arrangement  with  offsets 
has  the  farther  advantage  in  its  favor  of  not  allowing  the  rain 
water  to  lodge  in  the  joint,  if  the  offset  be  slightly  bevelled  off. 

353.  Workmen,  unless  narrowly  watched,  seldom  take  the  pains 
necessary  to  dress  the  beds  and  joints  accurately ;  on  the  con- 
trary, to  obtain  what  are  termed  close  joints,  they  dress  the  ioints 


Fig  7— Represents  a  section  of  a  wall  in  which  the 
face  is  of  cut  stone,  with  the  tails  of  the  blocki 
thinned  off,  and  the  backing  of  rubble 

A,  section  of  face  block. 

3,  rubble  backing. 


with  accuracy  a  few  inches  only  from  the  outward  surface,  and 
then  chip  away  the  stone  towards  the  back,  or  tail,  (Fig.  7,)  so 


110 


MASONRY 


that,  when  the  blocV.  is  set,  it  will  be  in  contact  with  the  adjacent 
stones,  only  through  rat  this  very  small  extent  of  bearing  surface. 
This  practice  is  objectionable  under  every  point  of  view ;  for, 
in  the  first  place,  it  gives  an  extent  of  bearing  surface,  which, 
being  generally  inadequate  to  resist  the  pressure  thrown  on  it, 
causes  the  block  to  splinter  off  at  the  joint ;  and  in  the  second 
place,  to  give  the  block  its  proper  set,  it  has  to  be  propped  be- 
neath by  small  bits  of  stone,  or  wooden  wedges,  an  operation 
termed  pinning-up,  or  under-pinning,  and  these  props,  causing 
the  pressure  on  the  block  to  be  thrown  on  a  few  points  of  the 
lower  surface,  instead  of  being  equally  diffused  over  it,  expose 
the  stone  to  crack. 

354.  When  the  facing  is  of  cut  stone,  and  the  backing  of  rub- 
ble, the  method  of  thinning  off  the  block  may  be  allowed  for  the 
purpose  of  forming  a  better  bond  between  the  rubble  and  ashlar ; 
but,  even  in  this  case,  the  block  should  be  dressed  true  on  each 
ioint,  to  at  least  one  foot  back  from  the  face.  If  there  exists  any 
cause,  which  would  give  a  tendency  to  an  outward  thrust  from 
the  back,  then,  instead  of  thinning  off  all  the  blocks  towards  the 
tail,  it  will  be  preferable  to  leave  the  tails  of  some  thicker  than 
the  parts  which  are  dressed. 

355.  Various  methods  are  used  by  builders  for  the  bond  of  cut 
stone.  The  system,  termed  headers  and  stretchers,  in  which  the 
vertical  joints  of  the  blocks  of  each  course  alternate  with  the  ver- 
tical joints  of  the  courses  above  and  below  it,  or  as  it  is  termed 
break  joints  with  them,  is  the  most  simple,  and  offers,  in  most  cases, 
all  requisite  solidity.  In  this  system,  (Fig.  8,)  the  blocks  of  each 
course  are  laid  alternately  with  their  greatest  and  least  dimensions 
to  the  face  of  the  wall ;  those  which  present  the  longest  dimen- 


Mall      1 

!        \o\        1 

II         II      1 

III         1 

!   1         II 

III         i 

*^\d$t@**d*t£*&JC"**ti 


«T 


7) 





Fig.  8— Represents  an  elevation  A,  end  view 
B,  and  plan  C,  of  a  wall  arranged  as  head- 
ers and  stretchers. 

a,  stretchers. 

b,  headers. 


*ion  along  the  face,  are  termed  stretchers ;  the  others,  headers 
If  the  header  reaches  from  the  face  to  the  back  of  the  wall,  it  is 


MASONRY. 


Ill 


termed  a  through ;  if  it  only  reaches  part  of  the  distance,  it  ia 
termed  a  binder.  The  vertical  joints  of  one  course  are  either 
just  o\  er  the  middle  of  the  blocks  of  the  next  course  below,  or 
else,  at  least  four  inches  on  one  side  or  the  other  of  the  vertica1 
joints  of  that  course  ;  and  the  headers  of  one  course  rest  as  nearly 
as  practicable  on  the  middle  of  the  stretchers  of  the  course  be- 
neath. If  the  backing  is  of  rubble,  and  the  facing  of  cut  stone,  a 
system  of  throughs  or  binders,  similar  to  what  has  just  been  ex- 
plained, must  be  used. 

By  the  arrangement  here  described,  the  facing  and  backing  of 
each  course  are  well  connected ;  and,  if  any  unequal  settling 
takes  place,  the  vertical  joints  cannot  open,  as  would  be  the  case 
were  they  in  a  continued  line  from  the  top  to  the  bottom  of  the 
mass  ;  as  each  block  of  one  course  confines  the  ends  of  the  two 
blocks  on  which  it  rests  in  the  course  beneath. 

356.  In  masses  of  cut  stone  exposed  to  violent  shocks,  as  those 
of  which  light-houses,  and  sea-walls  in  very  exposed  positions 
are  formed,  the  blocks  of  each  course  require  to  be  not  only  very 
firmly  united  with  each  other,  but  also  with  the  courses  above 
and  below  them.  To  effect  this,  various  means  have  been  used. 
The  beds  of  one  course  are  sometimes  arranged  with  projections 
(Fig.  9,)  which  fit  into  corresponding  indentations  of  the  next 
course.     Iron  cramps  in  the  form  of  the  letter  S,  or  in  any  other 


A 

i 

1     l 

■ 

1   :; 

s^^i 

1 

V 


S^i 


'-!**• 


F^ 


^x 


Fig.  9  —  Represents 
an  elevation  A, 
plan  B,  and  per- 
spective views  C 
and  D  of  two  of 
the  blocks  of  a  wall 
in  which  the  blocks 
are  fitted  with  in- 
dents, and  connect- 
ed with  bolts  and 
cramps  of  metal. 


shape  that  will  answer  the  purpose  of  giving  them  a  firm  hold  on 
the  blocks,  are  let  into  the  top  of  two  blocks  of  the  same  course 
at  a  vertical  joint,  and  are  firmly  set  with  melted  lead,  or  with 
bolts,  so  as  to  confine  the  two  blocks  together.  Holes  are,  in 
some  cases,  drilled  through  several  courses,  and  the  blocks  of 
these  courses  are  connected  by  strong  iron  bolts  fitted  to  the 
holes. 

The  most  noted  examples  of  these  methods  of  strengthening 
the  bond  of  cut  stone,  are  to  be  found  in  the  works  of  the  Romans 


112 


MASONRY. 


which  have  been  preserved  to  our  time,  and  in  two  celebrated 
modern  structures,  the  Eddy-stone  and  Bell-rock  light-houses  ir. 
Great  Britain.     (Fig.  10.) 


Fig.  10— Represents  the  manner  of  arranging  stone* 
of  the  same  course  by  dove-tail  joints  and  joggling, 
taken  from  a  horizontal  section  of  the  masonry  of 
the  Bell-rock  light-house. 


357.  The  manner  of  dressing  stone  belongs  to  the  stonecutter's 
art,  but  the  engineer  should  not  be  inattentive  either  to  the  accu- 
racy with  which  the  dressing  is  performed,  or  the  means  employed 
to  effect  it.  The  tools  chiefly  used  by  the  workman  are  the 
chisel,  axe,  and  hammer  for  knotting.  The  usual  manner  of  dress- 
ing a  surface,  is  to  cut  draughts  around  and  across  the  stone  with 
the  chisel,  and  then  to  use  the  chisel,  the  axe  with  a  serrated  edge, 
or  the  knotting  hammer,  to  work  down  the  intermediate  portions 
into  the  same  surface  with  the  draughts.  In  performing  this  last 
operation,  the  chisel  and  axe  should  alone  be  used  for  soft  stones, 
as  the  grooves  on  the  surface  of  the  hammer  are  liable  to  become 
choked  by  a  soft  material,  and  the  stone  may  in  consequence  be 
materially  injured  by  the  repeated  blows  of  the  workman.  In 
hard  stones  this  need  not  be  apprehended. 

In  large  blocks  which  require  to  be  raised  by  machinery,  a 
hole,  of  the  shape  of  an  inverted  truncated  wedge,  is  cut  to  receive 


Fig-  11— Represents  a  perspective 
view  A  of  a  block  of  stone  with 
draughts  around  the  edges  of  its 
faces,  and  the  intermediate  space 
axed,  or  knotted,  and  its  tackling 
for  hoisting:  also  the  common 
iron  lewis  B  with  its  tackling. 

a,  draughts  around  edge  of  block. 

b,  knotted  part  between  draughts. 

c,  iron  bolts  with  eyes  let  into  obliqua 
holes  cut  in  the  block. 

d  and  e,  chain  and  rope  tackling. 

n,  n,  side  pieces  of  the  lewis. 

o,  centre  piece  of  lewis  with  ey« 

fastened  to  n,  n  by  a  bolt. 
p,  iron  ring  for  attaching  tackling. 


a  small  iron  instrument  termed  a  lewis,  (Fig.  11,)  to  which  the 
rope  is  attached  for  suspending  the  block  ;  or  else  two  holes  arc 


MASONRY.  113 

cut  obliquely  into  the  block  to  receive  bolls  with  eyes  for  the 
same  purpose. 

When  a  block  of  cut  stone  is  to  be  laid,  the  first  point  to  be 
attended  to,  is  to  examine  the  dressing,  which  is  done  by  placing 
the  block  on  its  bed,  and  seeing  that  the  joints  fit  close,  and  the 
face  is  in  its  proper  plane.  If  it  be  found  that  the  fit  is  not  accu- 
rate, the  inaccuracies  are  marked,  and  the  requisite  changes  made. 
The  bed  of  the  course,  on  which  the  block  is  to  be  laid,  is  then 
thoroughly  cleansed  from  dust,  &c,  and  well  moistened,  a  bed 
of  thin  mortar  is  laid  evenly  over  it,  and  the  block,  the  lower  sur- 
face of  which  is  first  cleansed  and  moistened,  is  laid  on  the  mor- 
tar-bed, and  well  settled  by  striking  it  with  a  wooden  mallet. 
When  the  block  is  laid  against  another  of  the  same  course,  the 
joint  between  them  is  prepared  wjth  mortar  in  the  same  manner 
as  the  bed. 

358.  Rubble  Stone  Masonry.  With  good  mortar,  rubble 
work,  when  carefully  executed,  possesses  all  the  strength  and 
durability  required  in  structures  of  an  ordinary  character ;  and  it 
is  much  less  expensive  than  cut  stone. 

359.  The  stone  used  for  this  work  should  be  prepared  simply 
by  knocking  off  all  the  sharp,  weak  angles  of  the  block  ;  it  is  then 
cleansed  from  dust,  &c,  and  moistened,  before  placing  it  on  its 
bed.  This  bed  is  prepared  by  spreading  over  the  top  of  the  lower 
course  an  ample  quantity  of  good  ordinary-tempered  mortar,  into 
which  the  stone  is  firmly  imbedded.  The  interstices  between  the 
larger  masses  of  stone  are  filled  in,  by  thrusting  small  fragments, 
or  chippings  of  stone,  into  the  mortar.  Finally,  the  whole  course 
may  be  carefully  grouted  before  another  is  commenced,  in  order 
to  fill  up  any  voids  left  between  the  full  mortar  and  stone. 

360.  To  connect  the  parts  well  together,  and  to  strengthen  the 
*veak  points,  throughs  or  binders  should  be  used  in  all  the  courses ; 
and  the  angles  should  be  constructed  of  cut  or  hammered  stone. 
In  heavy  walls  of  rubble  masonry,  the  precaution,  moreover, 
should  be  observed,  to  lay  the  stones  on  their  quarry -bed ;  that 
is,  to  give  them  the  same  position,  in  the  mass  of  masonry,  that 
they  had  in  the  quarry  ;  as  stone  is  found  to  offer  more  resistance 
to  pressure  in  a  direction  perpendicular  to  the  nuarry-bed,  than 
in  any  other.  The  direction!)  of  the  lamina  in  stratified  stones, 
show  the  position  of  the  quairy-bed. 

361.  Hammered  stone,   or  dressed  rubble,  is  stone  roughly 
fasluoned  into  regular  masses  with  the  hammer.    The  same  pre 
cautions  must  be  taken  in  laying  this  kind  of  masonry,  as  in  the 
two  preceding. 

362.  Brick  Masonry.  With  good  brick  and  mortar,  this  ma 
sonry  offers  great  strength  and  durability,  arising  from  the  strong 
adhesion  between  the  mortar  and  brick. 

15 


114  MASONRY. 

363.  The  bond  used  in  brick  work  is  very  various,  depending 
on  the  character  of  the  structure.  The  most  usual  kinds  are 
Known  as  the  English  and  Flemish.  The  first  consists  in  ar- 
ranging the  courses  alternately,  entirely  as  headers  or  stretchers, 
the  bricks  through  the  course  breaking  joints.  In  the  second  the 
bricks  are  laid  as  headers  and  stretchers  in  each  course.  The 
first  is  stated  to  give  a  stronger  bond  than  the  last,  the  bricks  of 
which,  owing  to  the  difficulty  of  preventing  continuous  joints, 
either  in  the  same  or  different  courses,  are  liable  to  separate, 
causing  the  face  or  the  back  to  bulge  outward.  The  Flemish 
bond  presents  the  finer  architectural  appearance,  and  is  therefore 
preferred  for  the  fronts  of  edifices. 

364.  Timber  and  iron  have  both  been  used  to  strengthen  the 
bond  of  brick  masonry.  Among  the  most  remarkable  example." 
of  their  uses  are  the  well,  faced  in  brick,  forming  an  entrance  to 
the  Thames  Tunnel,  the  celebrated  work  of  Mr.  Brunei,  and  his 
experimental  arch  of  brick,  a  description  of  which  is  given  in  the 
Civil  Engineer  and  Architect's  Journal,  No.  6,  vol.  I.  In  both 
these  structures  Mr.  Brunei  used  pantile  laths  and  hoop  iron,  in 
the  interior  of  the  horizontal  courses,  to  connect  two  contiguous 
courses  throughout  their  length.  The  efficacy  of  this  method 
lias  been  farther  fully  tested  by  Mr.  Brunei,  in  experiments  made 
on  the  resistance  to  a  transversal  strain  of  a  brick  beam  bonded 
with  hoop  iron,  accounts  of  which,  and  of  experiments  of  a  like 
kind,  are  given  by  Colonel  Pasley  in  his  work  on  Limes,  Calca- 
reous Cements,  <xc. 

365.  The  mortar-bed  of  brick,  may  be  either  of  ordinary,  or 
thin-tempered  mortar ;  the  last,  however,  is  the  best,  as  it  makes 
closer  joints,  and,  containing  more  water,  does  not  dry  so  rapidly 
as  the  other.  As  brick  has  great  avidity  for  water,  it  would  al- 
ways be  well  not  only  to  moisten  it  before  laying  it,  but  to  allow 
it  to  soak  in  water  several  hours  before  it  is  used.  By  taking 
this  precaution,  the  mortar  between  the  joints  will  set  more  firmly 
than  when  it  imparts  its  water  to  the  dry  brick,  which  it  frequently 
does  so  rapidly  as  to  render  the  mortar  pulverulent  when  it  has 
dried. 

FOUNDATIONS. 

366.  The  term  foundation  is  used  indifferently  either-  for  the 
lower  courses  of  a  structure  of  masonry,  or  for  the  artificial 
arrangement,  of  whatever  character  it  may  be,  on  which  these 
courses  rest.  For  more  perspicuity,  the  term  bed  of  the  founda- 
tion will  be  used  in  this  work  when  the  latter  is  designated. 

367.  The  strength  and  durability  of  structures  of  masonry  de- 
pend essentially  up-?n  the  bed  of  the  foundation.  In  arranging 
this,  regard  must  be  had  not  only  to  the  permanent  efforts  which 


FOUNDATIONS.  115 

the  bed  may  have  to  support,  but  to  those  uf  an  accidental  cha- 
racter. It  should,  in  all  cases,  be  placed  so  far  below  the  surface 
of  the  soil  on  which  it  rests,  that  it  will  not  be  liable  to  be  un- 
covered, or  exposed ;  and  its  surface  should  not  only  be  normal 
to  the  resultant  of  the  efforts  which  it  sustains,  but  this  resultant 
should  intersect  the  base  of  the  bed  so  far  within  it,  that  the  por- 
tion of  the  soil  between  this  point  of  intersection  and  the  outward 
edge  of  the  base  shall  be*broad  enough  to  prevent  its  yielding 
from  the  pressure  thrown  on  it. 

368.  The  first  preparatory  step  to  be  taken,  in  determining  the 
kind  of  bed  required,  is  to  ascertain  the  nature  of  the  subsoil  on 
which  the  structure  is  to  be  raised.  This  may  be  done,  in  or- 
dinary cases,  by  sinking  a  pit ;  but  where  the  subsoil  is  composed 
of  various  strata,  and  the  structure  demands  extraordinary  pre- 
caution, borings  must  be  made  with  the  tools  usually  employed 
for  this  purpose. 

369.  With  respect  to  foundations,  soils  are  usually  divided 
into  three  classes : 

The  1st  class  consists  of  soils  which  are  incompressible,  or,  at 
least,  so  slightly  compressible,  as  not  to  affect  the  stability  of  the 
heaviest  masses  laid  upon  them,  and  which,  at  the  same  time,  do 
not  yield  in  a  lateral  direction.  Solid  rock,  some  tufas,  compact 
stony  soils,  hard  clay  which  yields  only  to  the  pick,  or  to  blast- 
ing, belong  to  this  class. 

The  2d  class  consists  of  soils  which  are  incompressible,  but 
require  to  be  confined  laterally,  to  prevent  them  from  spreading 
out.     Pure  gravel  and  sand  belong  to  this  class. 

The  3d  class  consists  of  all  the  varieties  of  compressible  soils  ; 
under  which  head  may  be  arranged  ordinary  clay,  the  common 
earths,  and  marshy  soils.  Some  of  this  class  are  found  in  a  more 
or  less  compact  state,  and  are  compressible  only  to  a  certain  ex- 
tent, as  most  of  the  varieties  of  clay  and  common  earth ;  others 
are  found  in  an  almost  fluid  state,  and  yield,  with  facility,  in  every 
direction. 

370.  To  prepare  the  bed  for  a  foundation  on  rock,  the  thick- 
ness of  the  stratum  of  rock  should  first  be  ascertained,  if  there  are 
any  doubts  respecting  it :  and  if  there  is  any  reason  to  suppose 
that  the-  stratum  has  not  sufficient  strength  to  bear  the  weight  of 
the  structure,  it  should  be  tested  by  a  trial  weight,  at  least  twice 
as  great  as  the  one  it  will  have  to  bear  permanently.  The  rock 
is  next  properly  prepared  to  receive  the  foundation  courses,  by 
levelling  its  surface,  which  is  effected  by  breaking  down  all  pro- 
ecting  points,  and  rilling  up  cavities,  either  with  rubble  masonry 
or  with  beton  ,  and  by  carefully  removing  any  portions  of  the  up 
per  stratum  which  present  indications  of  having  been  injured  by 
the  weather.    The  surface,  prepared  in  this  manner,  should,  more 


116  MASONRY. 

over,  be  perpendicular  to  the  direction  of  the  pressure  ;  if  this  ii 
vertical,  the  surface  should  be  horizontal,  and  so  for  any  othei 
direction  of  the  pressure.  Should  there,  however,  be  any  diffi- 
culty in  so  arranging  the  surface  as  to  have  it  normal  to  the  re- 
sultant of  the  pressure,  it  may  receive  a  position  such  that  one 
component  of  the  resultant  shall  be  perpendicular  to  it,  and  the 
other  parallel ;  the  latter  being  counteracted  by  the  friction  and 
adhesion  between  the  base  of  the  bed  £nd  the  surface  of  the  rock. 
It.  owing  to  a  great  declivity  of  the  surface,  the  whole  cannot  be 
brought  to  the  same  level,  the  rock  must  be  broken  into  steps,  in 
order  that  the  bottom  courses  of  the  foundation  throughout,  may 
rest  on  a  surface  perpendicular  to  the  direction  of  the  pressure. 
If  fissures  or  cavities  are  met  with,  of  so  great  an  extent  as  to 
render  the  filling  them  with  masonry  too  expensive,  an  arch  must 
then  be  formed,  resting  on  the  two  sides  of  the  fissure,  to  support 
that  part  of  the  structure  above  it. 

The  slaty  rocks  require  most  care  in  preparing  them  to  receive 
a  foundation,  as  their  top  stratum  will  generally  be  found  injured 
to  a  greater  or  less  depth  by  the  action  of  frost. 

371.  In  stony  earths  and  hard  clay,  the  bed  is  prepared  by 
digging  a  trench  wide  enough  to  receive  the  foundation,  and  deep 
enough  to  reach  the  compact  soil  which  has  not  been  injured  by 
the  action  of  frost :  a  trench  from  4  to  6  feet,  will  generally  be 
deep  enough  for  this  purpose. 

372.  In  compact  gravel,  and  sand,  where  there  is  no  liability 
to  lateral  yielding,  either  from  the  action  of  rain  or  any  other 
cause,  the  bed  may  be  prepared  as  in  the  case  of  stony  earths. 
If  there  is  danger  from  lateral  yielding,  the  part  on  which  the 
foundation  is  to  rest  must  be  secured  by  confining  it  laterally  by 
means  of  sheeting  piles,  or  in  any  other  way  that  will  offer  suffi- 
cient security. 

373.  In  laying  foundations  on  firm  sand,  a  further  precaution 
is  sometimes  resorted  lo,  of  placing  a  platform  on  the  bottom  of 
the  trench,  for  the  purpose  of  distributing  the  whole  weight  more 
uniformly  over  it.  This,  however,  seems  to  be  unnecessary ;  for 
if  the  bottom  courses  of  the  masonry  are  well  settled  in  their  bed, 
there  is  no  good  reason  to  apprehend  any  unequal  settling  from  the 
effect  of  the  superincumbent  weight :  whereas,  if  the  woai  of  the 
platform  should,  by  any  accident,  give  way,  it  would  leave  a  pari 
of  the  foundation  without  any  support. 

When  the  sand  under  the  bed  is  liable  to  injury  from  springs 
they  must  be  cut  off,  and  a  platform,  or,  still  better,  an  area  of 
beton  should  compose  the  bed,  and  this  should  be  confined  on  all 
sides  between  walls  of  stone,  or  beton  sunk  below  the  bottom  of 
the  bed. 

374.  If,  in  opening  \  trench  in  sand,  water  is  found  at  a  slight 


FOUNDATIONS. 


117 


depth,  and  in  such  quantity  as  to  impede  the  labors  of  the  work- 
men, and  the  trench  cannot  be  kept  dry  by  the  use  of  pumps  01 
scoops,  a  row  of  sheeting  piles  must  be  driven  on  each  side  of 
the  space  occupied  by  it,  somewhat  below  the  bottom  of  the  bed, 
the  sand  on  the  outside  of  the  sheeting  piles  be  thrown  out,  and 
its  place  filled  with  a  puddling  of  clay,  to  form  a  water-tight  en- 
closure round  the  trench.  The  excavation  for  the  bed  is  then 
commenced  ;  but  if  it  be  found  that  the  water  still  makes  rapidly 
at  the  bo  torn,  only  a  small  portion  of  the  trench  must  be  opened, 
and  after  me  lower  courses  are  laid  in  this  portion,  the  excavation 
will  be  g  rdually  effected,  as  fast  as  the  workmen  can  execute 
the  work  without  difficulty  from  the  water. 

375.  TK-  beds  of  foundations  in  compressible  soils  require  pe- 
culiar care  vs«rticularly  when  the  soil  is  not  homogeneous,  pre- 
senting more,  vp.sistance  to  pressure  in  one  point  than  in  another ; 
for,  in  that  o*\  it  will  be  very  difficult  to  guard  against  unequal 
settling. 

376.  In  ordi; i;-'  ^lay,  or  earth,  a  trench  is  dug  of  the  proper 
width,  and  deep  ene'Wi  to  reach  a  stratum,  beyond  the  action  of 
frost ;  the  bottom  of  r»0  trench  is  then  levelled  off  to  receive  the 
foundation.  This  mav  be  laid  immediately  on  the  bottom,  or 
else  upon  a  grillage  a.nA  f)c*form.  In  the  first  case,  the  stones 
forming  the  lowest  cour*e,  should  be  firmly  settled  in  their 
beds,  by  ramming  them  with  a  ~ery  heavy  beetle.  In  the  second 
a  timber  grating,  termed. a  sji*!,ln»C  (Fig.  12,)  which  is  formed 
of  a  course  of  heavy  beams  laid  1  "north wise  in  the  trench,  ano 
connected  firmly  by  cross  pieces  into  which  they  are  notched,  is 
firmly  settled  in  the  bed,  and  the  earth  i--  solidly  packed  between 
the  longitudinal  and  cross  pieces ;  a  f*c*mng  of  thick  planks, 
termed  a  platform,  is  then  laid  on  tne  gr<Ciffe,  to  receive  the 
lowest  course  of  the  foundation.     The  object  »    the  grillage,  and 


rf_i 


1 

c 

w 

Hfl 

ja 

a 

■fiftx 

A 

$& 

Fig.  1-2— Represents  the  arrangeme* 

platform  fitted  on  piles. 
A,  masonry, 
aa,  piles. 

b,  string  pieces. 

c,  cro6s  pieces. 
a,  capping  piece. 

e,  plattorm  of  plank. 


^Uaga  and 


platform,  is  to  diffuse  the  weight  more  uniformly  o*e. 
face  of  the  trench,  to  prevent  any  part  from  yielding. 


118.  MASONKf. 

377.  Repeated  failures  in  grillages  and  platfonns,  arising  either 
from  the  compression  of  the  woody  fibre,  or  from  a  transversa] 
strain  occasioned  by  the  subsoil  offering  an  unequal  resistance, 
have  impaired  confidence  in  their  efficacy.  Engineers  now  pre 
fer  beds  formed  of  an  area  of  beton,  as  offering  more  security  than 
any  bed  of  timber,  either  in  a  uniformly,  or  unequally  compressi- 
ble soil. 

378.  The  preparation  of  an  area  of  beton  for  the  bed  of  a 
foundation,  will  depend  on  the  circumstances  of  the  case.  In 
ordinary  cases  the  beton  is  thrown  into  the  trench,  and  carefully 
rammed  in  layers  of  6  or  9  inches,  until  the  mortar  collects  in  a 
semi-fluid  state  on  the  top  of  the  layer.  If  the  base  of  the  bed  is 
to  be  broader  than  the  top,  its  sides  must  be  confined  by  boards 
suitably  arranged  for  this  purpose.  Whenever  a  layer  is  left  in- 
complete at  one  end,  and  another  is  laid  upon  it,  an  offset  should 
be  left  at  the  unfinished  extremity,  for  the  purpose  of  connecting 
the  two  layers  more  firmly  when  the  work  on  the  unfinished  part 
is  resumed. 

Considerable  economy  may  be  effected,  in  the  quantity  of  be- 
ton required  for  the  bed,  by  using  large  blocks  of  stone  which 
should  be  so  distributed  throughout  the  layer,  that  the  beetle  will 
meet  with  no  difficulty  in  settling  the  beton  between  and  around 
the  blocks. 

When  springs  rise  through  the  soil  over  which  the  beton  is  to 
be  spread,  the  water  from  them  must  either  be  conveyed  off  by 
artificial  channels,  which  will  prevent  it  rising  through  the  mass 
of  beton  and  washing  out  the  lime  ;  or  else  strong  cloth,  prepared 
so  as  to  be  impermeable  to  water,  may  be  laid  over  the  surface 
of  the  soil  to  receive  the  bed  of  beton. 

The  artificial  channels  for  conveying  off  the  water  may  be 
formed  either  of  stone  blocks  with  semi-cylindrical  channels  cut 
in  them,  or  of  semi-cylinders  of  iron,  or  tiles,  as  may  be  most 
convenient.  A  sufficient  number  of  these  channels  should  be 
formed  to  give  an  outlet  to  the  water  as  fast  as  it  rises. 

An  impermeable  cloth  may  be  formed  of  stout  canvass,  pre- 
pared with  bituminous  pitch  and  a  drying  oil.  It  is  well  to  have 
the  cloth  doubled  on  each  side  with  ordinary  canvass  to  prevent 
accidents.  The  manner  of  settling  the  cloth  on  the  surface  of 
the  soil  must  depend  on  the  circumstances  of  the  case. 

Each  of  the  foregoing  expedients  for  preventing  the  action  of 
springs  on  an  area  of  beton,  has  been  tried  with  success.  When 
artificial  channels  are  used,  they  may  be  completely  choked  sub- 
sequently, by  injecting  into  them  a  semi-fluid  hydraulic  cement, 
and  the  action  of  the  springs  be  thus  destroyed. 

Foundation  beds  of  beton  are  frequently  made  without  exhaust- 
ing the  water  from  the  area  on  which  they  are  laid.     For  this 


FOUNDATIONS. 


119 


purpose  the  beton  is  thrown  in  layers  over  the  area,  by  using 
either  a  wooden  conduit  reaching  nearly  to  the  position  of  the 
layer,  or  else  by  placing  the  beton  (Fig  13)  in  a  box  by  which  it 
is  lowered  to  the  position  of  the  layer,  and  from  which  it  is  de- 
posited so  as  not  to  permit  the  water  to  separate  the  lime  from 
the  other  ingredients. 


Fig.  13— Represents  an  end 
view  A  of  a  semi-cylindri- 
cal box  for  lowering  beton 
in  water,  and  B  the  same 
view  of  the  box  when  open- 
ed to  let  the  beton  fall 
through, 
o,  hinge  around  which  the 
halves  of  the  box  open. 
B  a,  rope  tackling  for  lowering 
box. 

b,  pin,  or  catch  to  fasten  the 
two  parts  of  the  box. 

c,  cord  to  detach  the  pin  and 
open  the  box. 


Should  it  be  found  that  springs  boil  up  at  the  bottom,  it  must 
be  covered  with  an  impermeable  cloth. 

379.  In  marshy  soils,  the  principal  difficulty  consists  in  form- 
ing a  bed  sufficiently  firm  to  give  stability  to  the  structure,  owing 
to  the  yielding  nature  of  the  soil  in  all  directions. 

The  following  are  some  of  the  dispositions  that  have  been  tried 
with  success  in  this  case.  Short  piles  from  6  to  12  feet  long, 
and  from  6  to  9  inches  in  diameter,  are  driven  into  the  soil  as 
close  together  as  they  can  be  crowded,  over  an  area  considerably 
greater  than  that  which  the  structure  is  to  occupy.  The  heads 
of  the  piles  are  accurately  brought  to  a  level  to  receive  a  grillage 
and  platform ;  or  else  a  layer  of  clay,  from  4  to  6  feet  thick,  is 
laid  over  the  area  thus  prepared  with  piles,  and  is  either  solidly 
rammed  in  layers  of  a  foot  thick,  or  submitted  to  a  very  heavy 
pressure  for  some  time  before  commencing  the  foundations.  The 
object  of  preparing  the  bed  in  this  manner,  is  to  give  the  up- 
per stratum  of  the  soil  all  the  firmness  possible,  by  subjecting 
it  to  a  strong  compression  from  the  piles  ;  and  when  this  has 
been  effected,  to  procure  a  firm  bed  for  the  lowest  course  of  the 
foundation  by  the  grillage,  or  clay  bed ;  by  these  means  the 
whole  pressure  will  be  uniformly  distributed  throughout  the  en- 
tire area.  This  case  s  also  one  in  which  a  bed  of  beton  would 
replace,  with  great  advantage,  either  the  one  of  clay,  or  the 
grillage. 

The  purposes  to  which  the  short  piles  are  applied  in  this  case 
is  different  from  the  object  to  be  attained  usually  in  die  employ 


120  MASONRY. 

ment  of  piles  for  foundations  ;  which  is  to  transmit  the  weight  of 
the  structure  that  rests  on  the  piles,  to  a  firm  incompressible  soil, 
overlaid  by  a  compressible  one,  that  does  not  offer  sufficient 
firmness  for  the  bed  of  the  foundation. 

380.  When  a  firm  soil  is  overlaid  by  one  of  a  compressible 
character,  and  its  distance  below  the  surface  is  such  that  it  can  be 
reached  by  piles  of  ordinary  dimensions,  they  should  be  used  in 
preference  to  any  other  plan,  when  they  can  be  rendered  durable, 
on  account  of  their  economy  and  the  security  they  afford. 

To  prepare  the  bed  to  receive,  the  foundations  in  this  case, 
strong  piles  are  driven  at  equal  distances  apart,  over  the  entire 
area  on  which  the  structure  is  to  rest.  These  piles  are  driven, 
until  they  meet  with  a  firm  stratum  below  the  compressible  one, 
which  offers  sufficient  resistance  to  prevent  them  from  penetra- 
ting farther.  ' 

381.  Piles  are  generally  from  9  to  18  inches  in  diameter,  with 
a  length  not  above  20  times  the  diameter,  in  order  that  they  may 
not  bend  under  the  stroke  of  the  ram.  They  are  prepared  for 
driving,  by  stripping  them  of  their  bark,  and  paring  down  the 
knots,  so  that  the  friction,  in  driving,  may  be  reduced  as  much  as 
possible.  The  head  of  the  pile  is  usually  encircled  by  a  strong 
hoop  of  wrought  iron,  to  prevent  the  pile  from  being  split  by  the 
action  of  the  ram.  The  foot  of  the  pile  may  receive  a  shoe 
formed  of  ordinary  boiler  iron,  well  fitted  and  spiked  on ;  or  a 
cast-iron  shoe  of  a  suitable  form  for  penetrating  the  soil  may  be 
cast  around  a  wrought-iron  bolt,  by  means  of  which  it  is  fastened 
to  the  pile. 


Fig.  14— Represents  a  section  through  the  axis  of  a  cast-iron  shoe  and 
wrought-iron  bolt  for  a  pile 


382.  A  machine,  termed  a  pile  engine,  is  used  for  driving 
piles.  Tt  consists  essentially  of  two  uprights  firmly  connected 
at  top  by  a  cross  piece,  and  of  a  yam,  or  monkey  of  cast  iron,  for 
driving  the  pile  by  a  force  of  percussion.  Two  kinds  of  en- 
gines are  in  use  ;  the  one  termed  a  crab  engine,  from  the  ma- 
chinery used  to  hoist  the  ram  to  the  height  from  which  it  is  to 
fall  on  the  pile  ;  the  other  the  ringing  engine,  from  the  monkey 
being  raised  by  the  sudden  pull  of  several  men  upon  a  rope, 
by  which  the  ram  is  drawn  up  a  few  feet  to  descend  on  the 
pile. 

The  crab  engine  is  by  far  the  more  powerful  machine,  but  on 


FOUNDATIONS.  121 

this  account  is  inapplicable  in  some  cases,  as  in  the  driving  of 
cast-iron  piles,  where  the  force  of  the  blow  might  destroy  the 
pile  ;  also  in  long  slender  piles  it  may  cause  the  pile  to  spring  so 
much  as  to  prevent  it  from  entering  the  subsoil. 

The  manner  of  driving  piles,  and  the  extent  to  which  they  may 
be  forced  into  the  subsoil,  will  depend  on  local  circumstances.  It 
sometimes  happens  that  a  heavy  blow  will  effect  less  than  several 
slighter  blows,  and  that  piles  after  an  interval  between  successive 
volleys  of  blows,  can  with  difficulty  be  started  at  first.  In  some 
cases  the  pile  breaks  below  the  surface,  and  continues  to  yield  to 
the  blows,  by  the  fibres  of  the  lower  extremity  being  crushed. 
These  difficulties  require  careful  attention  on  the  part  of  the  en- 

f'neer.  Piles  should  be  driven  to  an  unyielding  subsoil.  The 
rench  civil  engineers  have,  however,  adopted  a  rule  to  stop 
the  driving  when  the  pile  has  arrived  at  its  absolute  stoppage, 
this  being  measured  by  the  farther  penetration  into  the  subsoil 
of  about  Toms  °f  an  inch,  caused  by  a  volley  of  thirty  blows 
from  a  ram  of  800  lbs.,  falling  from  a  height  of  5  feet  at  each 
blow. 

383.  If  the  head  of  a  pile  has  to  be  driven  below  the  level  to 
which  the  ram  descends,  another  pile,  termed  a  punch,  is  used 
for  the  purpose.  A  cast-iron  socket  of  a  suitable  form  embraces 
the  head  of  the  pile  and  the  foot  of  the  punch,  and  the  effect  of 
the  blow  is  thus  transmitted  through  the  punch  to  the  pile. 

384.  When  a  pile  from  breaking,  or  any  other  cause,  has  to  be 
drawn  out,  it  is  done  by  using  a  long  beam  as  a  lever  for  the  pur- 
pose ;  the  pile  being  attached  to  the  lever  by  a  chain,  or  rope 
suitably  adjusted. 

385.  The  number  of  piles  required,  will  be  regulated  by  the 
weight  of  the  structure.  An  allowance  of  1000  pounds  on  each 
square  inch  will  ensure  safety.  The  least  distance  apart,  at 
which  the  piles  can  be  driven  with  ease,  is  about  2\  feet  between 
their  centres.  If  they  are  more  crowded  than  this,  they  may 
force  each  other  up,  as  they  are  successively  driven.  When  this 
is  found  to  take  place,  the  driving  should  be  commenced  at  the 
centre  of  the  „  rea,  and  the  pile  should  be  driven  with  the  butt  end 
downward. 

386.  From  experiments  carefully  made  in  France,  it  appears 
that  piles  which  resist  only  in  virtue  of  the  friction  arising  from 
the  compression  of  the  soil,  cannot  be  subjected  with  safety  to  a 
load  greater  than  one  fifth  of  that  which  piles  of  the  same  dimen- 
sions will  safely  support  when  driven  into  a  firm  soil. 

387.  After  the  piles  are  driven,  they  are  sawed  off  to  a  level, 
to  receive  a  grillage  and  platform  for  the  foundation.  A  large 
beam,  termed  a  capping,  is  first  placed  on  the  heads  of  the  out- 
■i  le  row  of  piles,  to  which  it  is  fastened  by  means  of  wooden 

16 


122  MASONRY. 

pins,  or  tree-nans  driven  into  an  auger-hole,  made  through  the 
cap  into  the  head  of  each  pile.  After  the  cap  is  fitted,  longitudi 
nal  beams,  termed  string  pieces,  are  laid  lengthwise  on  the  heads 
of  each  row,  and  rest  at  each  extremity  on  the  cap,  to  which  they 
are  fastened  by  a  dove-tail  joint  and  a  wooden  pin.  Another  series 
of  beams,  termed  cross  pieces,  are  laid  crosswise  on  the  string 
pieces,  over  the  heads  of  each  row  of  piles.  The  cross  and  string 
pieces  are  connected  by  a  notch  cut  into  each,  so  that,  when  put 
together,  their  upper  surfaces  may  be  on  the  same  level,  and  they 
are  fastened  to  the  heads  of  the  piles  in  the  same  manner  as  the 
capping.  The  extremities  of  the  cross  pieces  rest  on  the  capping, 
and  are  connected  with  it,  like  the  string  pieces. 

The  platform  is  of  thick  planks  laid  over  the  grillage,  with 
the  extremity  of  each  plank  resting  on  the  capping,  to  which, 
and  to  the  string  and  cross  pieces,  the  planks  are  fastened  by 
nails. 

The  capping  is  usually  thicker  than  the  cross  and  string  pieces 
by  the  thickness  of  the  plank ;  when  this  is  the  case,  a  rabate, 
about  four  inches  wide,  must  be  made  on  the  inner  edge  of  the 
capping,  to  receive  the  ends  of  the  planks. 

388.  An  objection  is  made  to  the  platform  as  a  bed  for  the 
foundation,  owing  to  the  want  of  adhesion  between  wood  and 
mortar  ;  from  which,  if  any  unequal  settling  should  take  place, 
me  foundations  would  be  exposed  to  slide  off  the  platform.  To 
obviate  this,  it  has  been  proposed  to  replace  the  grillage  and  plat- 
form by  a  layer  of  beton  resting  partly  on  the  heads  of  the  piles, 
and  partly  on  the  soil  between  them.  This  means  would  furnisli 
a  firm  bed  for  the  masonry  of  the  foundations,  devoid  of  the  ob- 
jections made  to  the  one  of  timber. 

To  counteract  any  tendency  to  sliding,  the  platform  may  be 
inclined  if  there  is  a  lateral  pressure,  as  in  the  case,  for  example, 
of  the  abutments  of  an  arch. 

389.  In  soils  of  alluvial  formation,  it  is  common  to  meet  with 
a  stratum  of  clay  on  the  surface,  underlaid  with  soft  mud,  in 
which  case,  the  driving  of  short  piles  would  be  injurious,  as  the 
tenacity  of  the  stratum  of  clay  would  be  destroyed  by  the  oper- 
ation. It  would  be  better  not  to  disturb  the  upper  stratum 
in  this  case,  but  to  give  it  as  much  firmness  as  possible,  by 
ramming  it  with  a  heavy  beetle,  or  by  submitting  it  to  a  heavy 
pressure. 

390.  Piles  and  sheeting  piles  of  cast  iron  have  been  used  with 
complete  success  in  England,  both  for  the  ordinary  purposes  of 
cofferdams,  and  for  permanent  structures  for  wharfing.     The 

tiiles  have  been  cast  of  a  variety  of  forms  ;  in  some  cases  they 
lave  been  cast  hollow  for  the  purpose  of  excavating  the  soil 
within  the. pile  as  it  was  driven,  and  thus  facilitate  its  penetration 


FOUNDATIONS. 


123 


*nto  the  subsoil.    Fig.  15  represents  a  cross  section  of  one  of  the 
more  recent  arrangements  of  iron  piles  and  sheeting  piles. 


Fig.  15— Represents  a  horizontal  section  of  an  arrangement  of  piles  and  sheeting 
piles  of  cast  iron. 

a,  sheeting  pile  with  a  lap  e  to  cover  the  joint  between  it  and  the  next  sheeting 
pile. 

b,  piles  with  a  lap  on  each  side. 

c,  sheeting  pile  lapped  by  pile  and  sheeting  pile  next  it. 
a,  ribs  of  piles  and  sheeting  piles. 

891.  Sand  has  also  been  used  with  advantage  to  form  a  bed 
for  foundations  in  a  very  compressible  soil.  For  this  purpose 
a  trench  is  (Fig.  16)  excavated,  and  filled  with  sand  ;  the  sand 
being  spread  in  layers  of  about  9  inches,  and  each  layer  being 
firmly  settled  by  a  heavy  beetle,  before  laying  the  next.    If  water 


Fig.  16 — Represents  a  section  of  a  sand  foun- 
dation bed  and  the  masonry  upon  it. 
A,  sand  bed  in  a  trench. 
13,  masonry. 


should  make  rapidly  in  the  trench,  it  would  not  be  practicable  tc 
pack  the  sand  in  layers.     Instead,  therefore,  of  opening  a  trench, 


Fig.  17— Represents  a  section  of  a  foun- 
-     dation  bed  made  by  filling  holes  with 
sand. 

A,  holes  filled  with  sand. 

B,  masonry. 


holes  about  6  feet  deep,  and  6  inches  in  diameter,  (Fig.  17,< 


124  MASONRY. 

should  be  made,  by  means  of  a  short  pile,  as  close  together  af 
practicable  ;  when  the  pile  is  withdrawn  from  the  hole,  it  is  im- 
mediately filled  with  sa\d.  To  cause  the  sand  to  pack  firmly,  it 
should  be  slightly  moistened  before  placing  it  in  the  holes,  or 
trench. 

Sand,  when  used  in  this  way,  possesses  the  valuable  property 
of  assuming  a  new  position  of  equilibrium  and  stability,  should, 
the  soil  on  which  it  is  laid  yield  at  any  of  its  points.  Not  only 
does  this  take  place  along  the  base  of  the  sand  bed,  but  also  along 
the  edges,  or  sides,  when  these  are  enclosed  by  the  sides  of  the 
trench  made  to  receive  the  bed.  This  last  point  offers  also  some 
additional  security  against  yielding  in  a  lateral  direction.  The 
bed  of  sand  must,  in  all  cases,  receive  sufficient  thickness  to  cause 
the  pressure  on  its  upper  surface  to  be  distributed  over  the  entire 
base. 

392.  When,  from  the  fluidity  of  the  soil,  the  vertical  pressure 
of  the  structure  causes  the  soil  to  rise  around  the  bed,  this  action 
may  be  counteracted,  either  by  scooping  out  the  soil  to  some  depth 
around  the  bed  and  replacing  it  by  another  of  a  more  compact 
nature,  well  rammed  in  layers,  or  with  any  rubbish  of  a  solid 
character ;  or  else  a  mass  of  loose  stone  may  be  placed  over  the 
.surface  exterior  to  the  bed,  whenever  the  character  of  the  struc- 
ture will  warrant  the  expense. 

393.  Precautions  against  Lateral  Yielding.  The  soils  which 
have  been  termed  compressible,  strictly  speaking,  yield  only  by 
the  displacement  of  their  particles  either  in  a  lateral  direction,  or 
upward  around  the  structure  laid  upon  them.  Where  this  action 
arises  from  the  effect  of  a  vertical  weight,  uniformly  distributed 
over  the  base  of  the  bed,  the  preceding  methods  for  giving  per- 
manent stability  to  structure,  present  all  requisite  security.  But 
when  the  structure  is  subjected  also  to  a  lateral  pressure,  as  for 
example,  that  which  would  arise  from  the  action  of  a  bank  of 
earth  resting  against  the  back  of  a  wall,  additional  means  of  secu- 
rity are  demanded. 

One  of  the  most  obvious  expedients  in  this  case,  is  to  drive  a 
row  of  strong  square  piles  in  juxtaposition  immediately  in  contact 
with  the  exterior  edges  of  the  bed.  This  expedient  is,  however, 
only  of  service  where  the  pies  attain  either  an  incompressible 
soil,  or  one  at  least  firmer  than  that  on  which  the  bed  imme- 
diately rests.  For  otherwise,  as  is  obvious,  the  piles  only  serve 
to  transmit  the  pressure  to  the  yielding  soil  in  contact  with  them. 
But  where  they  are  driven  into  a  firm  soil  below,  they  gain  a 
fixed  point  of  resistance,  and  the  only  insecurity  they  offer  is 
either  by  the  rupture  of  the  piles,  from  the  cross  strain  upon 
them,  or  from  the  yielding  of  the  firm  subsoil,  from  the  same 
cause 


FOUNDATIONS. 


125 


In  case  the  piles  reach  a  firm  subsoil,  it  wil  oe  oesl  to  scoop 
out  the  upper  yielding  soil  before  driving  the  piles,  and  to  fill  in 
between  and  around  them  with  loose  broken  stone,  (Fig.  18.1 
This  will  give  the  piles  greater  stiffness,  and  effectually  preveni 
them  from  spreading  at  top. 


Fig.  18— Represents  the  manner  of  using 
loose  stone  to  sustain  piles  and  prevent 
them  from  yielding  laterally. 

A,  section  of  the  masonry. 

B,  loose  stone  thrown  around  the  piles  a 


When  the  piles  cannot  be  secured  by  attaining  a  firm  subsoil, 
:t  will  be  better  to  drive  them  around  the  area  at  some  distance 
f4-om  the  bed,  and,  as  a  farther  precaution,  to  place  horizontal 
buttresses  of  masonry  at  regular  intervals  from  the  bed  to  the 
piles.  By  this  arrangement,  some  additional  security  is  gained 
from  the  counter-pressure  of  the  soil  enclosed  between  the  bed 
and  the  wall  of  piles.  But  it  is  obvious  that  unless  the  piles  in 
this  case  are  driven  into  a  firmer  soil  than  that  on  which  the  struc- 
ture rests,  there  will  still  be  danger  of  yielding. 

In  using  horizontal  buttresses,  the  stone  of  which  they  are  con- 
structed should  be  dressed  with  care ;  their  extremities  near  the 
wall  of  piles  should  be  connected  by  horizontal  arches,  (Fig.  19,) 
to  distribute  the  pressure  more  uniformly  ;  and  where  there  is  an 
upward  pressure  of  the  soil  around  the  structure,  arising  from  its 
weight,  the  buttresses  ought  to  be  in  the  form  of  reversed  arches. 

In  buttresses  of  this  kind,  as  likewise  in  broad  areas  resting  on 
a  very  yielding  soil,  since  as  much  danger  is  to  be  apprehended 
from  their  breaking  by  their  own  weight  as  from  any  other  cause, 
it  must  be  carefully  guarded  against.  Something  may  be  done 
for  this  purpose  by  ramming  the  earth  around  the  structure  with 
a  heavy  beetle,  when  it  can  be  made  more  compact  by  this  means  ; 
or  else  a  part  of  the  upper  soil  may  be  removed,  and  be  replaced 
by  one  of  n  more  compact  nature  which  niy  be  rammed  m 
layers 


126 


MASONRY. 


Fig.  19— Represents  the  manner  of  pre- 
venting a  sustaining  wall  from  yielding 
laterally  to  a  thrust  behind  it,  by  using 
horizontal  buttresses  of  reversed  arches 
abutting  against  vertical  counter  arches. 

A,  vertical  section  of  wall,  buttresses,  and 
counter  arches. 

B,  plan  of  wall,  buttresses,  and  counter 
arches. 

a,  plan  of  wall. 

b,  section  of  do 

c,  buttresses. 

a,  counter  arches. 


The  following  methods,  where  they  can  be  resorted  to,  and 
where  the  character  of  the  structure  will  justify  the  expense,  have 
been  found  to  offer  the  best  security  in  the  case  in  question. 

When  the  bed  can  be  buttressed  in  front  with  an  embankment, 
a  low  counter-wall  (Fig.  20)  may  be  built  parallel  to  the  edge  of 
the  bed,  and  some  10  or  12  feet  from  it ;  between  this  wall  and 
the  bed  a  reversed  arch  connecting  the  two  may  be  built,  and  a 
surcharge  of  earth  of  a  compact  character  and  well  rammed,  may 
be  placed  against  the  counter  wall  to  act  by  its  counter  pressure 
against  the  lateral  pressure  upon  the  bed. 

Fig.  20 — Represents  the  man- 
ner of  buttressing  a  sustain- 
ing wall  in  front  by  the  ac- 
tion of  a  counter  pressure  of 
earth  transmitted  to  the  wall 
by  a  reversed  arch. 

a,  section  of  sustaining  wall. 

b,  section  of  sustaining  wall 
of  embankment  d. 

c,  section  of  reversed  arch. 

d,  section  of  embankment 
from  which  counter  pressure 
comes. 

e,  section  of  embankment  be- 
hind sustaining  wall. 

When  the  bed  cannot  be  buttressed  in  front,  as  in  quay  walls, 
a  grillage  and  platform  supported  on  piles  (Fig.  21)  may  be  built 
to  the  rear  from  the  back  of  the  wall,  for  the  purpose  of  support- 
ing the  embankment  against  the  back  of  the  wall,  and  preventing 
the  effect  which  its  pressure  on  the  subsoil  might  have  in  thrust- 
ing  forward  the  bed  of  the  foundation. 

In  addition  to  these  means,  land  ties  of  iron  will  give  great  ad 


FOUNDATIONS 


127 


ditional  security,  when  a  fixed  point  in  rear  of  the  wall  can  he 
found  to  attach  them  firmly. 


Fig.  21— Represents  the  manner  of  re- 
lieving a  sustaining  wall  from  th« 
lateral  action  caused  by  the  pressure 
of  an  embankment  on  the  subsoil  by 
means  of  a  platform  built  behind  the 
wall. 

A,  section  of  the  wall. 

B,  section  of  embankment. 
w    a,  piles  supporting  the  grillage  and  plat- 
form of  A. 

b,  loose  stone  forming  a  firm  bed  un- 
der the  platforms. 

c,  piles  supporting  the  platform  d  be- 
hind the  wall 


394.  Foundations  in  Water.  In  laying  foundations  in  water, 
two  difficulties  have  to  be  overcome,  both  of  which  require  great 
resources  and  care  on  the  part  of  the  engineer.  The  first  is  found 
in  the  means  to  be  used  in  preparing  the  bed  of  the  foundation ; 
and  the  second,  in  securing  the  bed  from  the  action  of  the  water, 
to  ensure  the  safety  of  the  foundations.  The  last  is,  generally, 
the  more  difficult  problem  of  the  two  ;  for  a  current  of  water  will 
gradually  wear  away,  not  only  every  variety  of  loose  soils,  but 
also  the  more  tender  rocks,  such  as  most  varieties  of  sand-stone, 
and  the  calcareous  and  argillaceous  rocks,  particularly  when  they 
are  stratified,  or  are  of  a  loose  texture. 

395.  To  prepare  the  bed  of  a  foundation  in  stagnant  water, 
the  only  difficulty  that  presents  itself  is  to  exclude  the  water  from 
the  area  on  which  the  structure  is  to  rest.  If  the  depth  of  water 
is  not  over  4  feet,  this  is  done  by  surrounding  the  area  with  an 
ordinary  water-tight  dam  of  clay,  or  of  some  other  binding  earth. 
For  this  purpose,  a  shallow  trench  is  formed  around  the  area,  by 
removing  the  soft,  or  loose  stratum  on  the  bottom  ;  the  foundation 
of  the  dam  is  commenced  by  filling  this  trench  with  the  clay,  and 
the  dam  is  made  by  spreading  successive  layers  of  clay  about  one 
foot  thick,  and  pressing  each  layer  as  it  is  spread,  to  render  it 
more  compact.  When  the  dam  s  completed,  the  water  is  pumped 
out  from  the  enclosed  area,  and  he  bed  for  the  foundation  is  pre- 
pared as  on  dry  land. 

396.  When  the  depth  of  stagnant  water  is  over  4  feet,  and  in 
running  water,  of  any  depth,  the  ordinary  dam  must  be  replaced 
by  the  coffer-dam.     This  construction  consists  of  two  rows  of 


128 


MASONRY. 


plank,  termed  sheeting  piles,  driven  into  the  soil  vertically,  form- 
ing thus  a  coffer  work,  between  which  clay  or  binding  earth, 
termed  the  puddling,  is  filled  in,  to  form  a  water-tight  dam  to  ex- 
clude the  water  from  the  area  enclosed. 

The  arrangement,  construction,  and  dimensions  of  coffer  dams 
depend  on  their  specific  object,  the  depth  of  water,  and  the  nature 
of  the  subscil  on  which  the  coffer-dam  rests. 

With  regard  to  the  first  point,  the  width  of  the  dam  between 
the  sheeting  piles  should  be  so  regulated  as  to  serve  as  a  scaffold- 
ing for  the  machinery  and  materials  required  about  the  work. 
This  is  peculiarly  requisite  where  the  coffer-dam  encloses  an  isola- 
ted position  removed  from  the  shore.  The  interior  space  enclosed 
by  the  dam  should  have  the  requisite  capacity  for  receiving  the 
bed  of  the  foundations,  and  such  materials  and  machinery  as  may 
be  required  within  the  dam. 

The  width,  or  thickness  of  the  coffer-dam,  by  which  is  under- 
stood the  distance  between  the  sheeting  piles,  should  be  sufficient 
not  only  to  be  impermeable  to  water,  but  to  form,  by  the  weight 
of  the  puddling,  in  combination  with  the  resistance  of  the  timber 
work,  a  wall  of  sufficient  strength  to  resist  the  horizontal  pressure 
of  the  water  on  tx.e  exterior,  when  the  interior  space  is  pumped 
dry.  The  resistance  offered  by  the  weight  of  the  puddling  to  the 
pressure  of  the  water  can  be  easily  calculated ;  that  offered  by 
the  timber  work  will  depend  upon  the  manner  in  which  the 
framing  is  arranged,  and  the  means  taken  to  stay,  or  buttress  the 
dam  from  the  enclosed  space. 

The  most  simple  and  the  usual  construction  of  a  coffer-dam 


Fig.  22 — Represents  a  sec- 
tion of  the  ordinary  cof- 
fer-dam. 

a,  main  piles. 

b,  wale,  or  6tring  pieces. 

c,  cross  pieces. 
a,  sheeting  piles. 

e,  guide  string  pieces  foi 
sheeting  piles. 

A,  puddlinir. 

B,  interior  space. 


(Fig.  22)  consists  in  driving  a  row  of  ordinary  straight  piles 
wound  the  area  to  be  enclosed,  placing  their  centre  lines  about  4 


FOUNDATIONS.  129 

feet  asunder.  A  second  row  is  driven  parallel  to  the  first,  the 
respective  piles  being  the  same  distance  apart ;  the  distance  be 
tween  the  centre  lines  of  the  two  rows  being  so  regulated  as  to 
leave  the  requisite  thickness  between  the  sheeting  piles  for  the 
dam.  The  piles  of  each  row  are  connected  by  a  horizontal  beam 
of  square  timber,  termed  a  string  or  wale  piece,  placed  a  foot  or 
two  above  the  highest  water  line,  and  notched  and  bolted  to  each 
pile.  The  string  pieces  of  the  inner  row  of  piles  is  placed  on  the 
side  next  to  the  area  enclosed,  and  those  of  the  outer  row  on  the 
outside.  Cross  beams  of  square  timber  connect  the  string  pieces 
of  the  two  rows,  upon  which  they  are  notched,  serving  both  to 
prevent  the  rows  of  piles  from  spreading  from  the  pressure  that 
may  be  thrown  on  them,  and  as  a  joisting  for  the  scaffolding.  On 
the  opposite  sides  of  the  rows,  interior  string  pieces  are  placed, 
about  the  same  level  with  the  exterior,  for  the  purpose  of  serving 
both  as  guides  and  supports  for  the  sheeting  piles.  The  sheeting 
piles  being  well  jointed,  are  driven  in  juxtaposition,  and  against 
the  interior  string  pieces.  A  third  course  of  string,  or  ribbon 
pieces  of  smaller  scantling  confine,  by  means  of  large  spikes,  the 
sheeting  piles  against  the  interior  string  pieces. 

As  has  been  stated,  the  thickness  of  the  dam  and  the  dimen- 
sions of  the  timber  of  which  the  coffer  work  is  made,  will  depend 
upon  the  pressure  due  to  the  head  of  water,  when  the  interior 
space  is  pumped  dry.  For  extraordinary  depths,  the  engineer 
would  not  act  prudently  were  he  to  neglect  to  verify  by  calcula- 
tion the  equilibrium  between  the  pressure  and  resistance  ;  but  for 
ordinary  depths  under  10  feet,  a  rule  followed  is  to  make  the 
thickness  of  the  dam  10  feet ;  and  for  depths  over  10  feet  to  give 
an  additional  thickness  of  one  foot  for  every  additional  depth  of 
three  feet.  This  rule  will  give  every  security  against  filtrations 
through  the  body  of  the  dam,  but  it  might  not  give  sufficient 
strength  unless  the  scantling  of  the  coffer  work  were  suitably  in- 
creased in  dimensions. 

In  very  deep  tidal  water,  coffer-dams  have  been  made  in  off- 
sets, by  using  three  rows  of  sheeting  piles  for  the  purpose  of 
giving  greater  thickness  to  the  dam  below  the  low-water  level. 
In  such  cases  strong  square  piles  closely  jointed  and  tongued  and 
grooved,  should  be  used  in  place  of  the  ordinary  sheeting  piles. 

Besides  providing  against  the  pressure  of  the  head  of  water, 
suitable  dimensions  must  be  given  to  the  sheeting  piles,  in  order 
that  they  may  sustain  the  pressure  arising  from  the  puddling  when 
the  interior  space  is  emptied  of  water.  This  pressure  against  the 
mterior  sheeting  piles  may  be  farther  increased  by  that  of  the  ex 
terior  water  upon  the  exterior  sheeting  piles,  should  the  pressure 
of  the  latter  be  greater  than  the  former.  To  provide  more  se- 
curely against  the  effect  of  these  pressures,  intermediate  string 

17 


130  MASONRY. 

pieces  may  be  placed  against  the  interior  row  of  piles  before  th* 
sheeting  piles  are  driven ;  and  the  opposite  sides  of  the  dam  on 
the  interior  may  be  buttressed  by  cross  pieces  reaching  across  the 
top  string  pieces,  and  by  horizontal  beams  placed  at  intermediate 
points  between  the  top  and  bottom  of  the  dam. 

The  main  inconvenience  met  with  in  coffer-dams  arises  from 
the  difficulty  of  preventing  leakage  under  the  dam.  In  all  cases 
the  piles  must  be  driven  into  a  firm  stratum,  and  the  sheeting 
piles  should  equally  have  a  firm  footing  in  a  tenacious  compact 
sub-stratum.  When  an  excavation  is  requisite  on  the  interior,  to 
uncover  the  subsoil  on  which  the  bed  of  the  foundation  is  to  be 
laid,  the  sheeting  piles  should  be  driven  at  least  as  deep  as  this 
point,  and  somewhat  below  it  if  the  resistance  offered  to  the 
driving  does  not  prevent  it. 

The  puddling  should  be  formed  of  a  mixture  of  tenacious  clay 
and  sand,  as  this  mixture  settles  better  than  pure  clay  alone. 
Before  placing  the  puddling,  all  the  soft  mud  and  loose  soil  be- 
tween the  sheeting  piles  should  be  carefully  extracted  ;  the  pud- 
dling should  be  placed  in  and  compressed  in  layers,  care  being 
taken  to  agitate  the  water  as  little  as  practicable. 

With  requisite  care  coffer-dams  may  be  used  for  foundations  in 
any  depth  of  water,  provided  a  water-tight  bottoming  can  be  found 
for  the  puddling.  Sandy  bottoms  offer  the  greatest  difficulty  in 
this  respect,  and  when  the  depth  of  water  is  over  5  feet,  extraor- 
dinary precautions  are  requisite  to  prevent  leakage  under  the 
puddling. 

When  the  depth  of  water  is  over  10  feet,  particularly  where 
the  bottom  is  composed  of  several  feet  of  soft  mud,  or  of  loose 
soil,  below  which  it  will  be  necessary  to  excavate  to  obtain  a  firm 
stratum  for  the  bed  of  the  foundation,  additional  precautions  will 
be  requisite  to  give  sufficient  support  to  the  interior  sheeting  piles 
against  the  pressure  of  the  puddling,  to  provide  against  leakage 
under  the  puddling,  and  to  strengthen  the  dam  against  the  pres- 
sure of  the  exterior  water,  when  the  interior  space  is  pumped  dry 
and  excavated.  The  best  means  for  these  ends,  when  the  local- 
ity will  admit  of  their  application,  is  to  form  the  exterior  of  the 
dam,  as  has  already  been  described,  by  using  piles  and  sheeting 
piles,  giving  to  the  latter  additional  points  of  support,  by  interme- 
diate string  pieces  between  the  one  at  top  and  the  bottom  of  the 
water ;  and  to  form  a  strong  framing  of  timber  for  a  support  to 
the  interior  sheeting  piles,  giving  to  it  the  dimensions  of  the  area 
to  be  enclosed.  The  frame-work  (Fig.  23)  may  be  composed  of 
upright  square  beams,  placed  at  suitable  distances  apart,  depend- 
ing on  the  strength  required,  upon  which  square  string  pieces  are 
bolted  at  suitable  distances  from  the  top  to  the  bottom,  the  bottom 
string  resting  on  the  surface  of  the  mud.     The  string  pieces 


FOUNDATIONS. 


131 


sen  mg  as  supports  for  the  sheeting  piles,  must  be  on  vhe  sides  of 
the  uprights  towards  the  puddling,  and  their  faces  in  the  same 


Fig.  23— Represents  & 
section  of  the  cof- 
fer-dam used  for 
the  Potomac  aque 
duct. 

«,  main  exterior  piles. 

b,  strong  square 
beams  correspond- 
ing to  a  on  which 
the  wales  n.  n  are 
notched  and  bolt- 
ed. 

c,  sheeting  piles. 

d,  top  wale  on  main 
piles. 

e,  cross  pieces. 

I,  guide  and  support- 
ing string  pieces  for 
sheeting  piles. 

oo,  horizontal  shores 
buttressing  opposite 
sides  of  dam. 

A,  puddling. 

B,  interior  space. 

C,  mud,  &c. 
1),  rock  bottom. 


vertical  plane.  Between  each  pair  of  opposite  uprights,  horizon- 
tal shores  may  be  placed  at  the  points  opposite  the  position  of  the 
string  pieces,  to  increase  the  resistance  of  the  dam  to  the  pressure 
of  the  water;  the  top  shores  extending  entirely  across  the  dam, 
and  being  notched  on  the  top  string  pieces.  The  interior  shores 
must  be  so  arranged  that  they  can  be  readily  taken  out  as  the 
masonry  on  the  interior  is  built  up,  replacing  them  by  other  shores 
resting  against  the  masonry  itself. 

397.  When  the  bed  of  a  river  presents  a  rocky  surface,  or  rock 
covered  with  but  a  few  feet  of  mud,  or  loose  soil,  cases  may  occur 
in  which  it  will  be  more  economical  and  equally  safe  to  lay  a  bed 
of  beton  without  exhausting  the  water  from  the  area  to  be  built 
on  ;  enclosing  the  area,  before  throwing  in  the  beton,  by  a  simple 
coffer  work  formed  of  a  strong  frame  work  of  uprights  and  hori- 
zontal beams  and  sheeting  piles.  The  frame  work  (Fig.  24)  in 
this  case  is  composed  of  uprights  connected  by  string  pieces  in 
pairs  ;  each  pair  is  notched  and  bolted  to  the  uprights,  a  sufficient 
interval  being  left  between  them  for  the  insertion  of  the  sheeting 
piles.  To  secure  the  frame  work  to  the  rock,  it  may  be  re- 
quisite to  drill  holes  in  the  rock  to  receive  the  foot  of  each  up- 
right. The  holes  may  be  drilled  by  means  of  a  long  iron  bar, 
termed  a  jumper,  which  is  used  for  this  purpose,  or  else  the  or- 
dinary diving  bell  may  be  employed.  This  machine  is  very  ser- 
viceable in  all  similar  constructions  where  an  examination  of  work 
under  water  is  requisite,  or  in  cases  where  it  is  necessary  to  lay 


132 


MASONRY. 


masonry  under  water.     The  frame  work  is  put  together  on  land, 
floated  to  its  position,  and  settled  upon  tl  .e  rock ;  the  sheeting 


Fig.  24— Represents  a  coffer  work  for  confining  beton. 

A,  section  of  coffer  work  and  beton. 

B,  plan  of  coffer  work. 

a,  a',  square  uprights  connected  by  horizontal  beams  b,  b 
bolted  to  them  in  pairs. 

c,  c',  sheeting  piles  inserted  between  the  beams  b,  b'  anc 
the  uprights  a,  a'. 

d,  d',  iron  rods  connecting  opposite  sides  of  coffer  work. 


piles  are  then  driven  into  close  contact  with  the  surface  of  the 
rock. 

398.  The  convenience  ano  economy  resulting  from  the  use  of 
beton  for  the  beds  of  structures  raised  in  water,  have  led  General 
Treussart  to  propose  a  plan  for  laying  beds  of  this  material,  and 
then  to  take  advantage  of  their  strength  and  impermeability  to  con- 
struct a  coffer-dam  upon  them,  in  order  to  carry  on  the  super- 
structure with  more  care.  To  effect  this,  the  space  to  be  occupied 
by  the  bed  (Fig.  25)  is  first  enclosed  by  square  piles,  driven  in 
juxtaposition  and  secured  at  top  by  a  string  piece.  The  mud  and 
loose  soil  are  then  scooped  from  the  enclosed  area  to  the  firm  soil 
on  which  the  bed  of  beton  is  to  be  laid.  The  bed  of  beton  is  next 
laid  with  the  usual  precautions,  and  while  it  is  still  soft  a  second 
row  of  square  piles  is  driven  into  it,  also  in  juxtaposition,  and  at 
a  suitable  distance  from  the  first  for  the  thickness  of  the  dam  - 


FOUNDATIONS.  133 

hese  are  also  secured  at  top  by  a  string  piece.    Cross  pieces  arc 


g.  25 — Represents  a  section  of  Gen* 
eral  Treussart's  dam. 
,  bed  of  beton. 

puddling  of  dam. 

masonry  of  structure, 
square  piles, 
wale  pieces, 
cross  pieces. 


notched  upon  the  string  pieces,  to  secure  the  rows  of  piles  and  form 
a  scaffolding.  An  ordinary  puddling  is  placed  in  between  the  rows 
of  piles,  and  the  interior  space  is  pumped  dry. 

Should  the  soil  under  the  bed  of  beton  be  permeable,  the  pres- 
sure of  the  water  on  the  base  of  the  bed  may  be  sufficient  to  raise 
the  bed  and  the  dam  upon  it,  when  the  water  is  taken  from  the 
interior  space.  A  proper  calculation  will  show  whether  this  dan- 
ger is  to  be  apprehended,  and  should  it  be,  a  provisional  weight 
must  be  placed  on  the  dam,  or  the  bed  of  beton,  before  exhaust- 
ing the  interior. 

399.  When  the  depth  of  water  is  great,  or  when,  from  the 
permeability  of  the  soil  at  the  bottom,  it  is  difficult  to  prevent 
leakage,  a  coffer-dam  may  be  a  less  economical  method  of  laying 
foundations  than  the  caisson.  The  caisson  (Fig.  26)  is  a  strong 
water-tight  vessel  having  a  bottom  of  solid  heavy  timber,  and 
vertical  sides  so  arranged  that  they  can  be  readily  detached  from 
the  bottom.  The  following  is  the  usual  arrangement  of  the  cais- 
son, it,  like  the  coffer-dam,  being  subject  to  changes  to  suit  it  to 
the  locality.  The  bottom  of  the  caisson,  serving  as  a  platform 
for  the  foundation  course  of  the  masonry,  is  made  level  and  of 
heavy  timber  laid  in  juxtaposition,  tiie  ends  of  the  beams  being 
confined  by  tenons  and  screw-bolts  to  longitudinal  capping  pieces 
of  larger  dimensions.  The  sides  of  the  box  are  usually  vertical, 
The  sides  are  formed  of  upright  pieces  of  scantling  covered  with 
thick  plank,  the  seams  being  carefully  calked  to  make  the  cais- 
son water-tight  The  lower  ends  of  the  uprights  are  inserted 
into  shallow  mortises  made  in  the  capping.     The  arrangement 


134 


MASONRY. 


for  detaching  the  sides,  is  effected  in  the  following  manner 
Strong  hooks  of  wrought  iron  are  fixed  to  the  bottom  of  the 


Fig.  26— Represents  a  cross  sec- 
tion and  interior  end  view  of 
a  caisson.  The  boards  in  this 
Fig.  are  represented  as  let 
into  grooves  in  the  vertica. 
pieces,  instead  of  being  nailed 
to  them  on  the  exterior. 

a,  bottom  beams  let  intc 
grooves  in  the  capping. 

b,  square  uprights  to  sustain 
the  boards. 

c,  cross  pieces  resting  on  b. 

d,  iron  rods  fitted  to  hooks  at 
bottom  and  nuts  at  top. 

e,  longitudinal  beams  to  stay 
the  cross  pieces  c. 

A,  section  of  the  masonry. 


caisson  at  the  sides  of  the  capping  piece,  corresponding  to  the 
points  where  the  uprights  of  the  sides  are  inserted  into  this  piece 
rieces  of  strong  scantling  are  laid  across  the  top  of  the  caisson, 
resting  on  the  opposite  uprights,  upon  which  they  are  notched. 
These  cross  pieces  project  beyond  the  sides,  and  the  projecting 
parts  are  perforated  by  an  auger-hole,  large  enough  to  receive  a 
bolt  of  two  inches  in  diameter.  The  object  of  these  cross  pieces 
is  twofold ;  the  first  is  to  buttress  the  sides  of  the  caisson  at  top 
against  the  exterior  pressure  of  the  water ;  and  the  second  is  to 
serve  as  a  point  of  support  for  a  long  bolt,  or  rod  of  iron,  with  an 
eye  at  the  lower  end,  into  which  the  hook  on  the  capping  piece 
is  inserted,  and  a  screw  at  top,  to  which  a  nut,  or  female  screw 
is  fitted,  and  which,  resting  on  the  cross  pieces  a;  a  point  of  sup- 
port, draws  the  bolt  tight,  and,  in  that  way,  attaches  the  sides 
and  bottom  of  the  caisson  firmly  together. 

A  bed  is  prepared  to  receive  the  bottom  of  the  caisson,  by  lev- 
elling the  soil  on  which  the  structure  is  to  rest,  if  it  be  of  a  suit- 
able character  to  receive  directly  the  foundation ;  or  by  driving 
large  piles  through  the  upper  compressible  strata  of  the  soil  to 
the  firm  stratum  beneath.  The  heads  of  the  piles  are  sawed  off 
on  a  level  to  receive  the  bottom  of  the  caisson. 

To  settle  the  caisson  on  its  bed,  it  is  floated  to  and  moored 
over  it ;  and  the  masonry  of  the  structure  is  commenced  and 
carried  up,  until  the  weight  grounds  the  caisson.  The  caisson 
should  be  so  contrived,  that  it  can  be  grounded,  and  afterwards 
raised,  in  case  that  the  bed  is  found  not  to  be  accurately  levelled 
To  effect  this,  a  sma'l  sliding  gate  should  be  placed  in  the  side 
of  the  caisson,  for  the  purpose  of  filling  it  with  water  at  pleasure 


FOUNDATIONS.  135 

Bv  means  of  this  gate,  the  caisson  can  be  filled  and  grounded, 
and,  by  closing  the  gate  and  pumping  out  the  water,  it  can  be  set 
afloat. 

After  the  caisson  is  settled  on  its  bed,  and  the  masonry  of  the 
structure  is  raised  above  the  surface  of  the  water,  the  sides  are 
detached,  by  first  unscrewing  the  nuts  and  detaching  the  rods 
and  then  taking  off  the  top  cross  pieces.  By  first  filling  the  cais- 
son with  water,  this  operation  of  detaching  the  sides  can  be  more 
easily  performed. 

400.  To  adjust  the  piles  before  they  are  driven,  and  to  prevent 
them  from  spreading  outward  by  the  operation  of  driving,  a  strong 
grating  of  heavy  timber,  formed  by  notching  cross  and  longitudi- 
nal pieces  on  each  other,  and  fastening  them  firmly  together,  may 
be  resorted  to.  This  grating  is  arranged  in  a  similar  manner  to 
a  grillage  ;  only  the  square  compartments,  between  the  cross  and 
string  pieces,  are  larger,  so  that  they  may  enclose  an  area  for  4 
or  9  piles  ;  and,  instead  of  a  single  row  of  cross  pieces,  the 
grating  is  made  with  a  double  row,  one  at  top,  the  other  at  the 
bottom,  embracing  the  string  pieces  on  which  they  are  notched. 

The  grating  may  be  fixed  in  its  position  at  any  depth  under 
water,  by  a  few  provisional  piles,  to  which  it  can  be  attached. 

401.  Where  the  area  occupied  by  a  structure  is  very  consider- 
able, and  the  depth  of  water  great,  the  methods  which  have  thus 
far  been  explained  cannot  be  used.  In  such  cases,  a  firm  bed 
is  made  for  the  structure,  by  forming  an  artificial  island  of  loose 
heavy  blocks  of  stone,  which  are  spread  over  the  area,  and  receive 
a  batter  of  from  one  perpendicular  to  one  base,  to  one  perpen- 
dicular and  six  base,  according  to  the  exposure  of  the  bed  to  the 
effects  of  waves.  This  bed  is  raised  several  feet  above  the  sur- 
face of  the  water,  according  to  the  nature  of  the  structure,  and 
the  foundation  is  commenced  upon  it. 

402.  It  is  important  to  observe,  that,  where  such  heavy  masses 
are  laid  upon  an  untried  soil,  the  structure  should  not  be  com- 
menced before  the  bed  appears  entirely  to  have  settled  ;  nor  even 
then,  if  there  be  any  danger  of  further  settling  taking  place  from 
the  additional  weight  of  the  structure.  Should  any  doubts  arise 
on  this  point,  the  bed  should  be  loaded  with  a  provisional  weight, 
somewhat  greater  than  that  of  the  contemplated  structure,  and 
this  weight  may  be  gradually  removed,  if  composed  of  other 
materials  than  those  required  for  the  structure,  as  the  work  pro- 
gresses. 

403.  To  give  perfect  security  to  foundations  in  running  water, 
the  soil  around  the  bed  must  be  protected  to  some  extent  from 
the  action  of  the  current.  The  most  ordinary  method  of  effect- 
ing this,  is  by  throwing  in  loose  masses  of  broken  stone  of  suffi- 
cient size  to  resist  the  force  of  the  current.     This  method  will 


136  MASONRY. 

give  all  required  security,  wheFe  the  soil  is  not  of  a  shifting  cha 
racter,  like  sand  and  gravel.  To  secure  a  soil  of  this  last  nature, 
it  will,  in  some  cases,  be  necessary  to  scoop  out  the  bottom  around 
the  bed  to  a  depth  of  from  3  to  6  feet,  and  to  fill  this  excavated 
part  with  beton,  the  surface  of  which  may  be  protected  from  the 
wear  arising  from  the  action  of  the  pebbles  carried  over  it  by  the 
current,  by  covering  it  with  broad  flat  flagging  stones. 

404.  When  the  bottom  is  composed  of  soft  mud  to  any  great 
depth,  it  may  be  protected  by  enclosing  the  area  with  sheeting 
piles,  and  then  filling  in  the  enclosed  space  with  fragments  of 
loose  stone.  If  the  mud  is  very  soft,  it  would  be  advisable,  in 
the  first  place,  to  cover  the  area  with  a  grillage,  or  with  a  layer 
of  brushwood  laid  compactly,  to  serve  as  a  bed  for  the  loose 
stone,  and  thus  form  a  more  stable  and  solid  mass. 

CONSTRUCTION  OF  MASONRY. 

405.  Under  this  head  will  be  comprised  whatever  relates  to  the 
manner  of  determining  the  forms  and  dimensions  of  the  most  im- 
portant elementary  components  of  structures  of  masonry,  together 
with  the  practical  details  of  their  construction.    • 

406.  Foundation  Courses.  As  the  object  of  the  foundations  is 
to  give  greater  stability  to  the  structure  by  diffusing  its  weight 
over  a  broad  surface,  their  breadth,  or  spread,  should  be  propor- 
tioned both  to  the  weight  of  the  structure  and  to  the  resistance 
offered  by  the  subsoil.  In  a  perfectly  unyielding  soil,  like  hard 
rock,  there  would  be  no  increase  of  stability  by  augmenting  the 
base  of  the  structure  beyond  what  would  be  strictly  necessary  to 
its  stability  in  a  lateral  direction ;  whereas  in  a  very  compressible 
soil,  like  soft  mud,  it  would  be  necessary  to  make  the  base  of  the 
foundation  very  broad,  so  that  by  diffusing  the  weight  over  a  great 
surface,  the  subsoil  may  offer  sufficient  resistance,  and  any  un- 
equal settling  be  obviated. 

407.  The  thickness  of  the  foundation  course  will  depend  on 
the  spread ;  the  base  is  made  broader  than  the  top  from  motives 
of  economy.     This  diminution  of  the  volume  (Fig.  27)  is  made 


Fig.  27— Section  of  foundation  courses  and  superstruc- 
ture. 

A,  batter. 

B,  offsets. 

C,  superstructure. 


either  in  steps,  termed  offsets,  or  else  by  giving  a  uniform  batter 
from  the  base  to  the  top.     The  latter  method  is  now  generally 


CONSTRUCTION  OF  MASONRY. 


137 


used ;  it  presents  equal  stability  with  the  former  with  a  smaller 
volume. 

When  the  foundation  has  to  resist  only  a  vertical  pressure,  an 
equal  batter  is  given  to  it  on  each  side  ;  but  if  it  has  to  resist  also 
a  lateral  effort,  the  spread  should  be  greater  on  the  side  opposed 
to  this  effort,  in  order  to  resist  its  tendency,  which  would  be  to 
cause  a  yielding  on  that  side. 

408.  The  bottom  course  of  the  foundations  is  usually  formed 
of  the  largest  sized  blocks,  roughly  dressed  off  with  the  hammer ; 
but  if  the  bed  is  compressible,  or  the  surfaces  of  the  blocks  are 
winding,  it  is  preferable  to  use  blocks  of  a  small  size  for  the  bot- 
tom course  ;  because  these  small  blocks  can  be  firmly  settled,  by 
means  of  a  heavy  beetle,  into  close  contact  with  the  bed,  which 
cannot  be  done  with  large  sized  blocks,  particularly  if  their  undej 
surface  is  not  perfectly  plane.  The  next  course  above  the  bottom 
one  should  be  of  large  blocks,  to  bind  in  a  firm  manner  the  smaller 
blocks  of  the  bottom  course,  and  to  diffuse  the  weight  more  uni 
formly  over  them. 

409.  When  a  foundation  for  a  structure  rests  on  isolated  sup- 
ports, like  the  pillars,  or  columns  of  an  edifice,  an  inverted  or 
counter-arch,  (Fig.  28,)  should  connect  the  top  course  of  the 
foundation  under  the  base  of  each  isolated  support,  so  that  the 
pressure  on  any  two  adjacent  ones  may  be  distributed  over  the 
bed  of  the  foundation  in  the  interval  between  them.  This  precau- 
tion is  obviously  necessary  only  in  compressible  soils.  In  incom- 
pressible soils  it  would  be  alone  requisite  to  carry  up  the  courses 
immediately  below  each  support  with  great  care,  to  present  a 
stable  bed  for  the  base  of  the  support. 


Fig.  28— Section  of  vertical 
supports  on  reversed  arch- 
es. 

A,  reversed  arch. 

B,  vertical  supports. 

C,  bed  of  stone. 


The  reversed  arch  is  also  used  to  give  greater  breadth  to  the 
foundations  of  a  wall  with  counterforts,  and  in  cases  where  an 
upward  pressure  from  water,  or  a  semi-fluid  soil  requires  to  be 
counteracted.  In  the  former  case  the  reversed  arches  are  turned 
under  the  counterforts  ;  in  the  latter  they  form  the  points  of  sup- 
port of  the  walls  of  the  structure. 

410.  The  angles  of  the  foundations  should  be  formed  of  the 
most  massive  blocks.     The  courses  should  be  carried  up  uni 

18 


138  MAS0NR1 

formlv  throughout  the  foundation,  to  p.-event  unequal   settling  ii 
the  mass. 

The  stones  of  the  top  course  of  the  foundation  shoii  i  be  sufE 
ciently  large  to  allow  the  course  of  the  superstructure  next  abo" 
to  rest  on  the  exterior  stones  of  the  top  course. 

411.  Hydraulic  mortar  should  be  used  for  the  foundations 
and  die  upper  courses  of  the  structure  should  not  be  commence 
until  the  mortar  has  partially  set  throughout  the  entire  found  -> 
tion. 

412.  Component  parts  of  Structures  of  Masonry  These  m  * 
be  divided  into  several  classes,  according  to  the  efforts  they  su* 
tain ;  their  forms  and  dimensions  depending  on  these  efforts. 

1st.  Those  which  sustain  only  their  own  weight,  and  are  nc 
liable  to  any  cross  strain  upon  the  blocks  of  which  they  ar 
formed,  as  the  walls  of  enclosures. 

2d.  Those  which,  besides  their  own  weight,  sustain  a  vertica 
pressure  arising  from  a  weight  borne  by  them,  as  the  walls  of  ed 
rices,  columns,  the  piers  of  arches,  &c. 

3d.  Those  which  sustain  lateral  pressures,  and  cross  strair 
upon  the  blocks,  arising  from  the  action  of  earth,  water,  frames 
or  arches. 

4th.  Those  which  sustain  a  vertical  upward,  or  downward 
pressure,  and  a  cross  strain,  as  areas,  lintels,  &c. 

5th.  Those  which  transfer  the  pressure  they  directly  receive 
to  lateral  points  of  support,  as  arches. 

413.  Walls  of  Enclosures.  Walls  for  these  purposes  may  be 
built  of  brick,  rubble,  or  dry  stone. 

Brick  walls  are  usually  built  vertically  upon  the  two  faces ; 
their  thickness  cannot  be  less  than  that  of  one  brick.  A  wall  of 
one  brick  and  a  half  thick  will  serve  for  any  length,  provided  the 
height  be  not  over  1 5  or  20  feet. 

Rubble  stone  walls  should  never  receive  a  thickness  less  than 
18  inches  when  the  two  faces  are  vertical.  Rondelet,  in  his  work 
VArt  de  Batir,  lays  down  a  rule  that  the  mean  thickness  of  both 
rubble  and  brick  walls  should  be  T'g  of  their  height. 

Dry  stone  walls  should  not  receive  a  less  thickness  than  two 
feet.  When  their  height  exceeds  12  feet,  their  mean  thickness 
should  be  ±  of  the  height. 

Stone  walls  are  usually  built  with  sloping  faces.  The  batter 
should  not  be  greater,  when  the  stones  are  cemented  with  mor- 
tar, than  one  base  to  six  perpendicular,  in  order  that  the  rain  may 
run  rapidly  from  the  surface,  and  that  the  wall  be  not  too  much 
exposed  to  decay  from  the  germination  of  seeds  which  may  lodge 
in  the  joints. 

The  batter  is  arranged  either  by  building  the  wall  in  offsets 
from  top  to  bottom,  or  by  a  uniform  surface.     In  either  case,  the 


CONSTRUCTION  OP  MASONRY.  139 

thickness  of  the  wall  at  top  should  not  be  less  than  from  8  to  1 2 
inches. 

When  a  wall  is  built  with  an  equal  batter  on  each  face,  and  the 
thickness  at  the  top  and  the  mean  thickness  are  fixed,  the  base  of 
the  wall,  or  its  thickness  at  the  bottom,  will  be  found  by  subtract- 
ing the  thickness  at  top  from  twice  the  mean  thickness.  This 
rule  evidently  makes  the  batter  of  the  wall  depend  upon  the  two 
preceding  dimensions. 

The  mean  thickness  of  long  walls  may  be  advantageously 
diminished  by  placing  counterforts,  or  buttresses,  upon  each  face 
at  equal  distances  along  the  line  of  the  wall.  These  are  spurs 
of  masonry  projecting  some  length  from  the  wall,  and  are  firmly 
connected  with  it  by  a  suitable  bond.  The  horizontal  section  of 
the  counterforts  may  be  rectangular ;  their  height  should  be  the 
same  as  that  of  the  wall. 

In  rubble  wall  the  counterforts  may  be  made  of  hammered,  or 
cut  stone.  In  addition  to  this  means  of  strengthening  walls,  hori- 
zontal courses,  or  chains  of  dressed  stone  may  be  advantageously 
used  from  distance  to  distance,  from  the  bottom  of  the  wall  up- 
ward. 

414.  Vertical  Supports.  These  consist  of  walls,  columns,  or 
pillars,  according  to  circumstances.  The  dimensions  of  the 
courses  of  masonry  which  compose  the  supports  should  be  regu- 
lated by  the  weight  borne.  If,  as  in  the  walls  of  edifices,  the 
resultant  of  the  efforts  sustained  by  the  wall  should  not  be  verti- 
cal, it  must  not  intersect  the  base  of  the  wall  so  near  the  outer 
edge,  that  the  stone  forming  the  lowest  course  would  be  in  danger 
of  being  crushed. 

In  broad  enclosed  spaces  covered  at  top,  the  dimensions  of  the 
wall  may  be  calculated  as  in  the  case  of  ordinary  enclosures,  and 
the  dimensions  thus  obtained  be  increased  in  proportion  to  the 
weight  to  be  borne. 

Cross  walls  between  the  exterior  walls,  as  the  partition  walls 
of  edifices,  should  be  regarded  as  counterforts  which  strengthen 
the  main  walls. 

415.  Areas.  The  term  area  is  applied  to  a  mass  of  masonry, 
usually  of  a  uniform  thickness,  laid  over  the  ground  enclosed  by 
the  foundations  of  walls.  It  seldom  happens  that  areas  have  an 
upward  pressure  to  sustain.  Whenever  this  occurs,  as  in  the 
case  of  the  bottoms  of  cellars  in  communication  with  a  head  of 
water  which  causes  an  upward  pressure,  the  thickness  and  ar- 
rangement of  the  area  should  be  regulated  to  resist  this  pressure. 
When  the  pressure  is  considerable,  an  area  of  uniform  thickness 
may  not  be  sufficiently  strong  to  ensure  safety ;  in  this  case  an 
inverted  arch  must  be  used. 

416.  Retaining,  or  Sustaining  Walls.     These  terms  are  ap- 


140 


MASONRY. 


pliea  to  walls  which  sustain  a  lateral  pressure  from  an  embank 
ment,  or  a  head  of  water. 

417.  Retaining  walls  may  yield  by  sliding  either  along  the 
base  of  the  foundation  courses,  or  along  one  of  the  horizontal 
joints,  or  by  rotation  about  the  exterior  edge  of  some  one  of  the 
horizontal  joints. 

418.  The  determination  of  the  form  and  dimensions  of  a  re- 
taining wall  for  an  embankment  of  earth  is  a  problem  of  consider- 
able intricacy,  and  the  mathematical  solutions  which  have  been 
given  of  it  have  generally  been  confined  to  particular  cases,  for 
which  approximate  results  alone  have  been  obtained ;  these,  how- 
ever, present  sufficient  accuracy  for  all  practical  purposes  within 
the  limits  to  which  the  solutions  are  applicable.  Among  the  many 
solutions  of  this  problem,  those  given  by  M.  Poncelet  of  the  Corps 
of  French  Military  Engineers,  in  a  Memoir  on  this  subject,  pub- 
lished in  the  Memorial  de  VOfficier  du  Genie,  No.  10,  present 
a  degree  of  research  and  completeness  which  peculiarly  charac- 
terize all  the  writings  of  this  gentleman,  and  have  given  to  his 
productions  a  claim  to  the  fullest  confidence  of  practical  men. 

The  following  formula,  applicable  to  cases  of  rotation  abdut  the 
exterior  edge  of  the  lowest  horizontal  joint,  are  taken  from  the 
memoir  above  cited. 

Calling  H,  the  height  BC  (Fig.  29)  of  a  wall  of  uniform  thick- 
ness, the  face  and  back  being  vertical. 

""^Mm\w 


Fig.  29— Represents  a  section  O  of  a  retaining  wall 
^  ^  P  "  '"-v  N         with  the  face  and  back  vertical. 

^  ^  i         "%  '     P»  section  of  the  embankment  above  the  wall. 

I    if  / 

h,  the  mean  height  CG  of  the  embankment,  retained  by  the  wall, 

above  the  top  of  the  wall. 
•;,  the  berm  DI,  or  distance  between  the  foot  of  the  embankment 

and  the  outer  edge  of  the  top  of  the  wall, 
a,  the  angle  between  the  line  of  the  natural  slope  BN  of  the  earth 

of  the  embankment  and  the  vertical  BG. 
/  =  cot.  a,  the  co-efficient  of  friction  of  the  earth  of  the  embank 

ment. 


CONSTRUCTION  OF  MASONRY. 


141 


w,  the  weight  of  a  cubic  foot  of  the  earth. 
«,•',  the  weight  of  a  cubic  foot  of  the  masonry  of  the  wall. 
b,  the  base  AB,  or  thickness  of  the  wall  at  bottom. 
Then, 

6=0.74  tan.  £a  V— ,  (A  +  1.126H)  +  0.0488A-  0.56c  tan.  a  (A 
w  ri 

_  0.6  £)(-*-  0.85). 

The  above  formula  gives  the  value  of  the  base  of  a  wall  with 
vertical  faces,  within  a  near  degree  of  approximation  to  the  true 
result,  only  when  the  values  of  the  quantities  which  enter  into  it 
are  confined  within  certain  limits.  These  limits  are  as  follows  : 
for  h,  between  0  and  H ;  c,  between  0  and  jH  ;  /,  between  0.6 
and  1.4,  which  correspond  to  values  of  a  of  70°  and  35°,  being 
in  the  one  case  the  angle  which  the  line  of  the  natural  slope  of 
very  fine  dry  sand  assumes,  and  in  the  other  of  heavy  clayey 
earth :  and  for  to,  between  w',  and  fiw'.  Besides  these  limits, 
the  formula  also  rests  on  the  assumption  that  the  excess  of  stabil- 
ity of  the  wall  over  that  of  a  strict  equilibrium  is  represented  by 
0.912  ;  or,  in  other  words,  that  the  moment  of  the  pressure  against 
the  wall  is  taken  0.912  greater  than  the  moment  of  strict  equi- 
librium between  it  and  the  wall.  This  excess  of  stability  given 
to  the  wall  supposes  an  excess  of  resistance  above  the  pressure 
against  it  equal  to  what  obtains  in  the  retaining  walls  of  Vauban, 
for  fortifications  which  have  now  stood  the  test  of  more  than  a 
century  with  security. 

419.  Having  by  the  preceding  formula  calculated  the  value  of 
6  for  a  vertical  wall,  the  base  b'  of  another  wall,  presenting  equal 
stability,  but  having  a  batter  on  the  face,  the  back  being  vertical, 


Fig.  30— Represents  a  section  O  of  •  retaining  wall  with 

a  sloping  lace  AD. 
P,  section  of  the  embankment. 


which  is  the  usual  form  of  the  cross  section  of  retaining  walls, 
can  be  calculated  from  the  following  notation  and  formula. 
Calling  (Fig.  30)  b'  the  base  of  the  si  oping  wall. 


142 


masonry. 


Ad 
n  =  tpr-j,  the  batter,  or  ratio  of  me  base  of  the  slope    o  the  per 

pendicular,  or  height  of  the  wail. 

Then, 

b'  =  b  +  r\nR. 

420.  With  regard  to  sliding  either  on  the  base  of  the  founda- 
tion courses,  or  on  the  bed  of  any  of  the  horizontal  joints  of  the 
wall,  M.  Poncelet  shows,  in  the  memoir  cited,  by  a  comparison 
of  the  results  obtained  from  calculations  made  under  the  suppo- 
sitions both  of  rotation  and  sliding,  that  no  danger  need  be  appre- 
hended from  the  latter,  when  the  dimensions  are  calculated  to 
conform  to  the  former,  so  long  as  the  limits  of  h  are  taken  between 
0  and  4H ;  particularly  if  the  precaution  be  taken  to  allow  the 
mortar  of  the  masonry  to  set  firmly  before  forming  the  embank- 
ment behind  the  wall. 

421.  Form  of  Section  of  Retaining  Walls.  Retaining  walls 
have  been  built  with  a  variety  of  forms  of  cross  section.  The  more 
usual  form  of  cross  section  is  that  in  which  the  back  of  the  wall 
H  built  vertically,  and  the  face  with  a  batter  varying  between  one 
base  to  six  perpendicular,  and  one  base  to  twenty-four  perpen- 
dicular. The  former  limit  having  been  adopted,  for  the  reasons 
already  assigned,  to  secure  the  joints  from  the  effects  of  weather  ; 
and  the  latter  because  a  wall  having  a  face  more  nearly  vertical, 
is  liable  in  time  to  yield  to  the  effects  of  the  pressure,  and  lean 
forward. 

422.  The  most  advantageous  form  of  cross  section  for  econo- 
my of  masonry  is  the  one  (Fig.  31)  termed  a  leaning  retaining 


Fig.  31— Represents  a  section  O  of  a  leaning  retaining 
wall  with  a  sloping  face  AD  and  the  back  BC  coun- 
ter-sloped. 


wall.  The  counter  slope,  or  reversed  batter  of  the  back  of  the 
wall,  should  not  be  less  than  six  perpendicular  to  one  base.  In 
this  case  strength  requires  that  the  perpendicular  let  fall  from  the 
centre  of  gravity  of  the  section  upon  the  base,  should  fall  so  far 
within  the  inner  edge  of  the  base,  that  the  stone  of  the  bottom 


CONSTRUCTION  OF  MASONRY. 


43 


course  of  the  foundation  may  present  sufficient  surface  to  :ear 
the  pressure  upon  it. 

423.  Walls  with  a  curved  batter  (Fig.  32)  both  upon  the  face 
and  back,  have  been  used  in  England,  by  some  engineers,  for 
quays.     They  present  no  peculiar  advantages  in  strength  over 


Fig.  32— Represents  a  section  A  of  a  wall  with  a 
curved  face  and  back,  and  an  elevation  B  of  th« 
counterforts. 

C,  water. 

D,  embankment  behind  the  wall. 
a,  fender  beams  of  timber 


walls  with  plane  faces  and  backs,  and  require  particular  care  in 
arranging  the  bond,  and  fitting  the  stones  or  bricks  of  the  face. 

424.  Measures  for  increasing  the  Strength  of  Retaining 
Walls.  These  consist  in  the  addition  of  counterforts,  in  the  use 
of  relieving  arches,  and  in  the  modes  of  forming  the  embank- 
ment. 

425.  Counterforts  give  additional  strength  to  a  retaining  wall 
m  several  ways.  By  dividing  the  whole  line  of  the  wall  into 
shorter  lengths  between  each  pair  of  counterforts,  they  prevent 
the  horizontal  courses  of  the  wall  from  yielding  to  the  pressure 
of  the  earth,  and  bulging  outward  between  the  extremities  of  the 
walls  ;  by  receiving  the  pressure  of  the  earth  on  the  back  of  the 
counterfort,  instead  of  on  the  corresponding  portion  of  the  back  of 
the  wall,  its  effect  in  producing  rotation  about  the  exterior  foot  of 
the  wall  is  diminished ;  the  sides  of  the  counterforts  acting  as 
abutments  to  the  mass  of  earth  between  them  may,  in  the  case  of 
sand,  or  like  soil,  cause  the  portion  of  the  wall  between  the  coun- 
terforts to  be  relieved  from  a  part  of  the  pressure  of  the  earth 
oehind  it,  owing  to  the  manner  in  which  the  particles  of  sand  be- 
come buttressed  against  each  other  when  confined  laterally,  and 
offer  a  resistance  to  pressure. 

426.  The  horizontal  section  of  counterforts  may  be  either 
rectangular,  or  trapezoidal.  Wher  placed  against  the  back  of  a 
wall,  the  rectangular  form  offers  the  greater  stability  in  the  case 
of  rotation,  and  is  more  economical  in  construction  ;  the  trape 


144 


MASONRY. 


eoidal  form  gives  a  broader  and  therefore  a  firmer  connection  be- 
tween the  wall  and  counterfort  than  the  rectangular,  a  point  oi 
some  consideration  where,  from  the  character  of  the  materials,  the 
strength  of  this  connection  must  mainly  depend  upon  the  strength 
of  the  mortar  used  for  the  masonry. 

427.  Counterforts  have  been  chiefly  used  by  military  engineers 
for  the  retaining  walls  of  fortifications,  termed  revetements.  In 
regulating  their  form  and  dimensions,  the  practice  of  Vauban  has 
been  generally  followed,  which  is  to  make  the  horizontal  section 
of  the  counterfort  trapezoidal,  making  the  height  of  the  trapezoid 
ef  (Fig.  33,)  which  corresponds  to  the  length  of  the  counterfort, 
two  tenths  of  the  height  of  the  wall  added  to  two  feet,  the  base  of 
the  trapezoid  ah  corresponding  to  the  junction  of  the  counterfort 
and  back  of  the  wall,  one  tenth  of  the  height  added  to  two  feet, 
and  the  side  cd  which  corresponds  to  the  back  of  the  counterfort 
equal  to  two  thirds  of  the  base  ab.     The  counterforts  are  placed 

RfcS-lst^vv.*^  «f  Su,  St  curving 

d.tY-x.1'  ^    oj*  I'avi  1 
itsta-rvct.  frovn.  $Q.t 


'<— Represents  a  section  A1,  and  plan  D  of  a  wall,  and 


Fig.  33- 
an  elevation  6,  and  plan  E  of  a  trapezoidal  counterfort. 


from  15  to  18  feet  from  centre  to  centre  along  the  back  of  the 
wall,  according  to  the  strength  required. 

428.  In  adding  counterforts  to  walls,  the  practice  has  generally 
been  to  regard  them  only  as  giving  additional  stability  to  the  wall, 
and  not  as  a  means  of  diminishing  its  volume  of  masonry  of 
which  the  addition  of  the  counterforts  ought  to  admit.  Considered 
in  this  last  point  of  view,  the  problem  for  determining  both  the 
suitable  dimensions  of  the  counterforts  and  the  thickness  of  the 
corresponding  wall,  is  one  of  very  considerable  mathematical 
difficulty,  whose  solution  must  repose  upon  assumptions  made  as 


CONSTRUCTION  OF  MASONRY. 


145 


to  the  manner  in  which  the  portions  of  the  wall  between  the 
counterforts  would  be  likely  to  yield  to  the  pressure  upon  them, 
the  support  which  they  receive  from  the  two  counterforts  at  their 
extremities,  and  the  stability  which  the  counterforts  add  to  the 
entire  system  in  preventing  rotation. 

429.  Relieving  Arches  are  so  termed  from  their  preventing  a 
portion  of  the  embankment  from  resting  against  the  back  of  the 
wall,  and  thus  relieving  it  from  a  part  of  the  pressure.  They 
consist  (Fig.  34)  of  one  or  more  tiers  of  brick  arches  built  upon 
counterforts,  which  act  as  the  piers  of  the  arches. 


Fig.  34— Represents  a  section  M  and  an  ele- 
vation N  of  a  wall  and  relieving  arches  in 


wall, 
the  arches  through  their 


^~-      vation  IN  ot  a  w 
f  three  tiers. 

A,  section  of  the  x 
I  c,  c,  c,  sections  of 

\s^~      crowns. 


d,  d,  interior  elevations  of  counterforts  serv- 
ing as  piers  of  the  arches. 

e,  e,  interior  end  elevations  of  arches. 


In  arranging  a  combination  of  relieving  arches  and  their  piers, 
the  latter,  like  ordinary  counterforts,  are  placed  about  18  feet 
apart  between  their  centre  lines  ;  their  length  should  be  so  regu- 
lated that  the  earth  behind  them  resting  on  the  arches,  and  falling 
under  them  with  the  natural  slope,  shall  not  reach  the  wall  be- 
tween the  arch  and  the  foot  of  the  back  of  the  wall  below  the  arch. 
The  thickness  of  the  arches,  as  well  as  that  of  the  counterforts, 
will  depend  upon  the  weight  which  the  arches  sustain.  The 
dimensions  of  the  wall  will  be  regulated  by  the  decreased  pres 
sure  against  it  caused  by  the  action  of  the  arches,  and  the  point 
at  which  this  pressure  acts. 

430.  Whenever  it  becomes  necessary  to  form  the  embankment 
before  the  mortar  of  the  retaining  wall  has  had  time  to  set  firmly, 
various  expedients  may  be  employed  to  relieve  the  wall  from  the 
pressure  which  the  embankment,  if  formed  of  loose  earth,  would 
throw  upon  it.  The  portion  of  the  embankment  next  to  the  wall 
may  be  of  a  compact  binding  earth  placed  in  layers  inclining 
downward  from  the  back  of  the  wall,  and  well  rammed  ;  or  of  a 
stiff  mortar  made  either  of  clay,  or  sand,  with  about  ^Tth  in  bulk 
of  lime.  Instead  of  bringing  the  embankment  directly  against 
the  back  of  the  wall,  dry  stone,  or  fascines  may  be  laid  in  to  a 
suitable  depth  back  from  the  wall  for  the  same  purpose.  The 
precaution,  however,  of  allowing  the  mortar  to  set  firmly  before 
Forming  the  embankment,  should  never  be  omitted  except  in  cases 
of  extreme  urgency,  and  then  the  bond  of  the  masonry  should  be 

19 


1 46  MASONRY. 

arranged  With  peculiar  care,  to  prevent  disjunction  aloi.g  any  ol 
the  horizontal  joints. 

431.  Walls  built  to  sustain  a  pressure  of  water  should  be  regu- 
lated in  form  and  dimensions  like  the  retaining  walls  of  embank- 
ments. The  problem  in  this  case  is  one  of  less  difficulty  thai! 
in  the  other,  from  the  greater  simplicity  of  the  mathematical 
formula  for  the  pressure  of  water.  The  buoyant  effort  of  the 
water  must  be  taken  into  account  in  this  calculation,  whenever  the 
masonry  is  so  placed  as  to  be  partially  immersed  in  the  water. 

432.  Heavy  walls,  and  even  those  of  ordinary  dimensions, 
when  exposed  to  moisture,  should  be  laid  in  hydraulic  mortar. 
Grout  has  been  tried  in  laying  heavy  rubble  walls,  but  with  de- 
cided want  of  success,  the  successive  drenchings  of  the  stone 
causing  the  sand  to  separate  from  the  lime,  leaving  when  dry  a 
weak  porous  mortar.  When  the  stone  is  laid  in  full  mortar,  grout 
may  be  used  with  advantage  over  each  course,  to  fill  any  voids 
left  in  the  mass. 

433.  Beton  has  frequently  been  used  as  a  filling  between  the 
back  and  facing  of  water-tight  walls ;  it  presents  no  advantage 
over  walls  of  cut,  or  rubble  stone  laid  in  hydraulic  mortar,  and 
causes  unequal  settling  in  the  parts,  unless  great  care  is  taken  in 
the  construction 

434.  When  a  wejght,  arising  from  a  mass  of  masonry  or  earth, 
rests  upon  two  or  more  isolated  supports,  that  portion  of  it  which 
is  distributed  over  the  space,  or  bearing  between  any  two  of  the 
supports,  may  be  borne  by  a  block  of  stone,  termed  a  lintel,  laid 
horizontally  upon  the  supports,  by  a  combination  of  blocks  termed 
a  plate-bande,  so  arranged  as  to  resist,  without  disjunction,  the 
pressure  upon  them  ;  or  by  an  arch. 

435.  Lintel.  Owing  to  the  slight  resistance  of  stone  to  a  cross 
strain,  and  to  shocks,  lintels  of  ordinary  dimensions  cannot  be 
used  alone  with  safety,  for  bearings  over  six  feet.  For  wider 
bearings,  a  slight  brick  arch  is  thrown  across  the  bearing  above 
the  lintel,  and  thus  relieves  it  from  the  pressure  of  the  parts 
above. 

436.  Plate-bande.  The  plate-bande  is  a  combination  of  blocku 
cut  in  the  form  of  truncated  wedges.  From  the  form  of  the 
blocks,  the  pressure  thrown  upon  them  causes  a  lateral  pressure 
which  must  be  sustained  either  by  the  supports,  or  by  some  other 
arrangement,  (Fig.  35.) 

The  plate-bande  should  be  used  only  for  narrow  bearings,  as 
the  upper  edges  of  the  blocks  at  the  acute  angles  are  liable  to 
splinter  from  the  pressure.  If  the  bearing  exceeds  10  feet,  the 
plate-bande  should  be  relieved  from  the  pressure  by  a  brick  arch 
•bove  it.  Additional  means  of  strengthening  the  plate-bande  are 
©meiimes  used  by  forming  a  broken  joint  between  the  blocks,  or 


CONSTRUCTION  OF  MASONRY. 


147 


by  a  projection  made  on  the  face  of  one  block  to  fit  into  a  cor< 
responding  indent  in  the  adjacent  one,  or  by  connecting  the  blocks 
with  iron  bolts. 

c 

[  Fig.  35— Represents  a  cross 
section  of  a  plate-bande, 
showing  the  manner  in 
which  the  voussoirs  A,  A 
and  B  are  cut  and  con- 
nected by  metal  cramps. 
ab,  tie  of  wrought  iron  lor 
the  plate-bande  fastened 
to  the  bolts  cd,  let  into 
the  piers  of  the  plate 
bande. 

When,  from  any  cause,  the  supports  cannot  be  made  suffi- 
ciently strong  to  resist  the  lateral  pressure  of  the  plate-bande,  the 
extreme  blocks  must  be  united  by  an  iron  bar,  termed  a  tie,  suit- 
ably arranged  to  keep  the  blocks  from  yielding. 

437.  Arches.  The  arch  is  a  combination  of  wedge-shaped 
blocks,  termed  arch  stones,  or  voussoirs,  truncated  towards  the 
angle  of  the  wedges  by  a  curved  surface  which  is  usually  normal 
to  the  surfaces  of  the  joints  between  the  blocks.  This  inferior 
surface  of  the  arch  is  termed  the  intrados,  or  soffit.  The  upper, 
or  outer  surface  of  the  arch  is  termed  the  extrados,  or  back, 
.'Fig.  36.) 


.46. 


L. 

/    e     1 

\  «} 

_ftL_ 

*n 

— 

r- 

...Q 


W/////M//////. ///////////// 

Fig.  30— Represents  an  elevation  31  of  the  head  of  a  right  cylindrical  arch, 
and  a  section  N  through  the  crown  of  the  arch  A,  with  an  elevation  B  of 
the  soffit  and  the  face  C  of  the  abutment. 

ab,  span  of  the  arch. 

dc-  rise. 

acb,  curve  of  the  intrados. 

e,  e,  voussoirs  forming  ring  ceursee  of  heads. 

f,  key  stone. 

g,  cushion  stone  of  abutment. 
mn,  crown  of  the  arch. 

op,  springing  line. 

438.  The  extreme  blocks  of  the  arch  rest  against  lateral  sup- 
ports, termed  abutments,  which  sustain  both  the  vertical  pressure 
arising  from  the  weight  of  the  arch  stones,  and  the  weight  of 
whatever  lies  upon  them  ;  also  the  lateral  pressure  caused  by  the 
action  of  the  arch. 

439.  In  a  range,  or  series    f  .rches  placed  side  by  side,  the 


148 


MASONRY, 


extreme  supports  are  termed  the  abutments,  the  inte.mediate  sup 
ports  which  sustain  the  intermediate  arches  and  the  halves  of  the 
two  extreme  ones  are  termed  piers.  When  the  size  of  the  arches 
is  the  same,  and  their  springing  lines  are  in  the  same  horizontal 
plane,  the  piers  receive  no  other  pressure  but  that  arising  from 
the  weight  of  the  arches. 

440.  Arches  are  classified,  from  the  form  of  the  soffit,  intd 
cylindrical,  conical,  conoidal,  warped  annular,  groined,  clois- 
tered, and  domes.  They  are  also  termed  right,  oblique,  or  askew, 
and  rampant,  from  their  direction  with  respect  to  a  vertical,  or 
horizontal  plane. 

441.  Cylindrical  Arch.  This  is  the  most  usual  and  the  sim- 
plest form  of  arch.  The  soffit  consists  of  a  portion  of  a  cylindri- 
cal surface.  When  the  section  of  the  cylinder  perpendicular  to 
the  axis  of  the  arch,  termed  a  right  section,  cuts  from  the  surface 
a  semi-circle,  the  arch  is  termed  a  full  centre  arch ;  when  the 
section  is  an  arc  less  than  a  semi-circle,  it  is  termed  a  segment 
arch;  when  the  section  gives  a  semi-ellipse,  it  is  termed  an 
elliptical  arch;  when  the  section  gives  a  curve  resembling  a 
semi-ellipse,  formed  of  arcs  of  circles  tangent  to  each  other,  the 
arch  is  termed  an  oval,  (Fig.  37,)  or  basket  handle,  and  is  called 


'-'•'-v.-.-...* 


Fig.  37— Represents  an  oval  curve  ol 
three  centres,  the  arcs  of  which  arc 
each  60°. 

DB,  span  of  the  curve. 

AC,  rise. 

P,  O,  and  R,  centres  of  the  arcs  of 
60°. 


I' 


a  curve  of  three,  five,  &c.  centres,  according  to  the  number  ot 
arcs,  which  must  be  odd  to  obtain  a  curve  symmetrical  with 
respect  to  the  vertical  line  bisecting  it ;  when  the  section  is  that 
of  two  arcs  of  circles  intersecting  at  the  middle  point  of  the  curve, 
it  is  termed  a  pointed,  or  an  obtuse  or  surbased  arch,  (Figs.  3S 
and  39,)  according  as  the  angle  between  the  arcs  at  their  inter- 
section is  acute,  or  obtuse. 

A  cylindrical  arch  is  denominated  a  right  arch  when  it  is  ter 
minated  by  two  planes,  termed  the  heads  of  the  arch,  perpendicu 


CONSTRUCTION  OF  MASONRY. 


149 


iar  to  the  axis  of  the  arch ;  oblique,  or  askew,  when  the  heads 
are  oblique  to  the  axis  ;  and  rampant  when  the  axis  of  the  arch 
is  oblique  to  the  horizontal  plane. 


Fig.  38— Represents  the  half  of  a  pointed  cum 

of  four  centres. 
nb,  half  span. 
be,  rise. 
m  and  n,  centres  of  the  half  curve  ae. 


\ 


\ 


\ 


Fig.  39— Represents  the  half  of  an  obtuse  or 

Based  curve  of  four  centres. 
ab,  half  span. 
be,  rise. 
m  and  n,  centres  of  the  half  curve  dc. 


442.  The  chord  of  the  curve  of  right  section  (see  Fig.  36)  is 
termed  the  span  of  the  arch,  its  versed  sine  the  rise  of  the  arch. 
When  the  heads  of  the  arch  are  oblique  to  the  axis,  the  chord  of 
the  oblique  section  made  by  the  plane  of  the  heads  is  termed  the 
span  of  the  askew  section.  The  lines  of  the  soffit  corresponding 
to  the  extremities  of  the  span  are  termed  the  springing  lines  of 
the  arch ;  the  top  portion,  or  line  of  the  soffit,  is  termed  the 
crown.  The  U  >  stone  of  the  crown  the  key  stone.  The  line 
drawn  through  tlie  middle  point  of  the  span  at  the  extremities  of 
the  arch,  is  termed  the  axis  of  the  arch.* 

443.  The  form  of  right  section  will  depend  upon  the  purposes 
which  the  arch  is  to  serve,  the  locality,  and  the  style  of  architec- 
ture employed.  When  the  rise  is  less  than  half  the  span,  the 
arch  is  weaker  than  in  cither  the  full  centre,  or  where  the  rise  La 

*  See  Xote  C,  Appendix. 


150  MASONRY. 

greater  than  half  the  span.   The  methods  of  ^escribing  the  various 
curves  of  right  section  will  be  explained  in  the  Appendix. 

444.  The  same  general  principle  is  followed  in  arranging  the 
joints  and  bond  of  the  masonry  of  arches,  as  in  other  structures 
of  cut  stone.  The  surfaces  of  the  joints  should  be  normal  to  the 
surface  of  the  soffit,  and  the  surfaces  of  any  two  systems  of  joints 
should  be  normal  to  each  other  at  their  lines  of  intersection.  These 
conditions,  with  respect  to  the  joints,  will  be  satisfied  by  tracing 
upon  the  soffit  its  lines  of  least  and  greatest  curvature,  and  taking 
the  edges  of  one  series  of  joints  to  correspond  with  one  of  these 
systems  of  lines,  and  the  edges  of  the  other  series  with  the  other 
system,  the  surfaces  of  the  joints  being  formed  by  the  surfaces 
normal  to  the  soffit  along  the  respective  lines  in  question.  When- 
ever the  surface  of  the  soffit  belongs  to  any  of  the  families  of 
geometrical  surfaces,  the  joints  will  be  thus  either  plane,  or  de- 
velopable surfaces.  In  the  cylindrical  arch,  for  example,  the 
edges  of  one  series  of  joints  will  correspond  to  the  elements  of 
the  cylindrical  surface,  while  those  of  the  other  will  correspond 
to  the  curves  of  right  section,  the  former  answering  to  the  line 
of  least,  and  the  latter  of  greatest  curvature.  The  surfaces  of 
the  joints  will  all  be  plane  surfaces,  and,  being  normal  to  the 
soffit  along  the  lines  in  question,  will  be  normal  also  to  each 
other. 

445.  In  full  centre  and  segment  arches,  the  voussoirs  are 
usually  made  of  the  same  breadth,  estimated  along  the  curve  of 
right  section.  The  planes  of  the  joints  of  each  course  of  vous 
soirs  between  the  heads  of  the  arch  are  made  continuous,  (see 
Fig.  36,)  each  of  these  courses  being  termed  a  string  course,  and 
their  joints  coursing  joints.  The  planes  of  the  joints  along  the 
curves  of  right  section  are  not  continuous,  but  break  joints ;  the 
stones  which  correspond  to  two  consecutive  series  of  these  joints 
being  termed  a  ring  course,  and  its  joints  heading  joints.  By 
this  combination  of  the  ring  and  string  courses,  the  fitting  of  the 
blocks,  the  settling  of  the  courses,  and  the  bond  are  arranged  in 
the  best  manner. 

440.  In  the  other  forms  of  right  section  of  cylindrical  arches, 
it  may  not.  in  many  cases,  be  practicable  to  give  the  voussoirs 
the  sime  breadth,  owing  to  the  variable  curvature  of  the  right  sec- 
tion ;  but  the  same  arrangement  is  followed  for  the  ring  and  string 
courses. 

447.  Ir  oblique  cylindrical  aiJies,  when  the  obliquity  is  but 
slight.  T»o  change  will  be  required  in  the  arrangement  of  the 
courses  and  joints ;  but  when  the  angle  between  the  heads  and 
the  axis  i?  considerably  less  than  a  right  angle,  the  ring  courses 
at  the  extremities  of  the  aiii  would  have  what  is  termed  a  false 
bearing,  that  is,  the  pressure  upon  their  coursing  joints  would 


CONSTRUCTION  OF  MASONRY  151 

not  be  transmitted  in  the  direction  of  the  pressure  to  the  fixed 
lateral  supports,  and  therefore  these  portions  of  the  arch  would 
be  insecure.  To  obviate  these  defects,  as  well  as  the  unequal 
bearing  upon  the  lateral  supports  in  such  case,  arrangements  of 
the  coursing  and  heading  joints  have  been  devised,  by  which  a 
better  bond  is  obtained,  and  the  total  pressure  from  the  voussoira 
thrown  upon  the  abutments. 

One  method  for  this  purpose  has  been  mostly  used  in  England, 
and  consists  in  placing  the  edges  of  the  heading  and  coursing 
joints  along  spira.  lines  of  the  cylindrical  soffit  which  intersect 
each  other  at  right  angles.  The  directing  spirals  for  the  heading 
joints  (Fig.  40)  being  taken  parallel  to  the  one  which  is  drawn  con 


Fig.  40 — Represents  an  ele- 
vation A  of  the  head  and 
of  a  part  of  the  soffit  B  of 
an  oblige  cylindrical  arch 
with  spiral  joints. 

«,  voussoirs  of  cut  stone. 

c,  c,  bottom  course  of  stone 
voussoirs  cut  to  receive  the 
brick  courses. 

C,  face  of  the  abutment. 

D,  ends  of  the  abutments. 


necting  the  extreme  points  of  the  askew  curve  of  the  head  ;  those 
for  the  coursing  being  traced  perpendicular  to  the  former.  The 
joints  being  normal  to  the  soffit  along  the  spirals,  will  be  helicoi- 
dal  surfaces.  This  method  palliates  only  to  some  extent  the 
weakness  of  the  bond  in  the  courses  near  the  heads,  and  giving 
a  considerable  dip  to  the  coursing  joints  at  the  extremities  of  the 
abutments  which  make  an  acute  angle  with  their  faces,  it  presents 
here  also  a  weak  point.  It  possesses  an  important  advantage, 
however,  in  permitting  the  soffit  ends  of  the  string  courses  to  be 
of  equal  breadth  throughout,  and  therefore  allows  the  method  tc 
be  adapted  as  well  to  brick  as  cut  stone.  To  bring  the  coursing 
joints  to  correspond  exactly  with  the  divisions  of  the  ring  courses 
of  the  heads,  it  may  be  necessary,  in  some  cases,  to  shift  the 
spirals  of  the  coursing  joints  slightly,  in  making  the  drawings  for 
the  arch.  The  end  blocks  of  the  string  courses  which  rest  upon 
the  abutment,  or  else  the  top  course  of  the  abutment,  must  be 
suitably  cut  to  correspond  to  the  direction  of  the  heading  joints 
and  that  of  the  horizontal  courses  of  the  abutment. 

448.  A  second  method,  in  use  among  the  French  engineers, 
consists  in  n  aking  the  heading  joints  plane  surfaces  and  parallel 
to  the  heads  of  the  arch,  and  in  taking  for  the  edges  of  the  coursing 
joints  (Fig.  41)  the  trajectories  traced  on  the  soffit  perpendicular 
to  the  edges  of  the  heading  joints.     The  surfaces  of  the  coursing 


152 


MASONRY'. 


joints  are  made  normal  to  the  soffit.  B\  nis  plan  sjme  of  the 
defects  of  the  former  are  remedied,  but  it  has  the  disadvantage 

Fig.  41  —  Represents 
an  elevation  of  the 
head  and  a  portion 
of  the  soffit  of  an 
oblique  cylindrical 
arch,  with  the 
edges  of  the  cours- 
ing joints  forming 
trajectories  at  right 
angles  to  the  edges 
of  heading  joints 
parallel  to  the 
curves  of  the  heads 
of  the  arch. 

The  letters  refer  to 
same  parts  as  in 
Fig.  40. 

of  giving  an  unequal  breadth  to  the  soffit  ends  of  the  voussoirs, 
and  therefore  is  inapplicable  to  brick  arches.  The  curves  of  the 
trajectories  and  the  coursing  joints  are  of  more  difficult  construc- 
tion than  in  the  first  method. 

449.  Cylindrical,  groined,  and  cloistered  arches  are  formed  by 
the  intersections  of  two  or  more  cylindrical  arches.  The  span 
of  the  arches  may  be  different,  but  the  rise  is  the  same  in  each. 
The  axes  of  the  cylinders  will  be  in  the  same  plane,  and  they  may 
intersect  under  any  angle. 

The  groined  arch  (Fig.  42)  is  formed  by  removing  those  por- 


*v 

M 

n 

\          l 
i 

ft 

V 

Fig.  42— Represents  the  plan  of  the  soffit 
and  the  right  sections'M  and  N  of  the 
cylinders  lorming  a  groined  arch. 

aa,  pillars  supporting  the  arch. 

be,  groins  of  the  sollit. 

am,  mn,  edges  of  coursing  joints. 

A,  key  stone  of  the  two  arches  formed  of 
one  block. 

B,  B,  groin  stones  of  one  block  below  the 
key  stone  forming  a  part  of  each  arch 


iions  of  each  cylinder  which  lie  under  the  other  and  between 
their  common  curves  of  intersection  ;  thus  forming  a  projecting, 
or  salient  edge  on  the  soffit  along  these  curves. 

The  cloistered  arch  (Fig.  43)  is  formed  by  removing  those  por- 
tions of  each  cylinder  which  are  above  the  other  and  exterior  to 
theii  common  intersection,  forming  thus  re-entering  angles  along 
the  same  lines. 

450.  The  planes  of  the  joints  in  both  of  these  arches  are  placed 
in  the  same  manner  as  in  the  simple  cylindrical  arch.     The  inne? 


CONSTRUCTION  OF  MASONRY. 


15? 


edges  of  the  corresponding  course  of  voussoirs  in  each  arch  are 
placed  in  the  same  plane  parallel  to  that  of  the  axes  of  the  cvlin- 

M 


Fig.  43— Represents  a  section  M  of  the  vciiseoirs  and 
an  elevation  of  the  soffit  of  a  cloistered  arch,  with 
a  plan  N  of  the  soffit. 

A,  A,  voussoirs. 

win,  edge  of  coursing  joint. 

o,  o,  edges  of  heading  joints 

H,  B,  abutments  of  the  arches. 

acb,  curve  of  the  groin. 

C,  C,  groin  stones  of  one  block. 


ders.  The  portions  of  the  soffit  in  each  cylinder,  corresponding 
to  each  course  of  voussoirs,  which  form  either  the  groin  in  the 
one  case,  or  the  re-entering  angle  in  the  other,  are  cut  from  a 
single  stone,  to  present  no  joint  along  the  common  intersection  of 
the  arches,  and  to  give  them  a  firmer  bond. 

451.  Conical  arches  are  of  rare  application.  When  used,  the 
same  general  principles  with  respect  to  the  joints  and  bond  apply 
to  them.  The  surfaces  of  one  set  of  joints  will  be  planes  passed 
through  the  elements  of  the  cone  and  normal  to  the  soffit ;  the 
other  will  be  conical,  or  other  surfaces,  likewise  normal  to  the 
soffit  and  passing  through  the  curves  of  least  curvature. 

452.  When  the  spans  at  the  two  ends  of  an  arch  are  unequal, 
but  the  rise  is  the  same,  then  the  soffit  of  the  arch  is  made  of  a 
conoidal  surface.  The  curves  of  right  section  at  the  two  ends 
may  be  of  any  figure,  but  are  usually  taken  from  some  variety 
of  the  elliptical,  or  oval  curves.  The  soffit  is  formed  by  moving 
a  line  upon  the  two  curves,  and  parallel  to  the  plane  containing 
their  spans. 

The  conoidal  arch  belongs  to  the  class  with  warped  soffits.  A 
variety  of  warped  surfaces  may  be  used  for  soffits  according  to 
circumstances ;  the  joints  and  the  bond  depending  on  the  gener- 
ation of  the  surface. 

453.  In  arranging  the  joints  in  conoidal  arches,  the  heading 
joints  are  contained  in  planes  perpendicular  to  the  axis  of  the 
arch.     The  coursing  joints  are  also  formed  of  plane  surfaces,  so 

20 


154 


MASONRY. 


arranged  that  the  portion  of  the  joint  corresponding  to  each  block 
is  formed  by  a  plane  normal  to  the  conoid  at  the  middle  point  of 
the  lower  edge  of  the  block.  In  this  way  the  joints  of  the  string 
course  will  not  be  formed  of  continuous  surfaces.  To  make 
them  so,  it  would  be  necessary  to  give  them  the  form  of  warped 
surfaces,  which  present  more  difficulty  in  their  mechanical  exe 
cution,  and  not  sufficient  advantages  over  the  method  just  ex- 
plained to  compensate  for  having  them  continuous. 

454.  The  annular  arch  is  formed  by  revolving  the  plane  of  a 
semi-circle,  or  semi-oval,  or  other  curve,  about  a  line  drawn  with- 
out the  figure  and  parallel  to  the  rise  of  the  arch,  (Fig.  44  )    One 


Fig.  44— Repre- 
sents a  plan 
M  of  the  abut- 
ments A  and 
B,  and  the 
soffit  C  of  an 
annular  arch. 

N,  right  section 
of  the  arch. 

a,  position  of 
vertical  axis 
around  which 
the  section  N 
is  revolved. 


series  of  joints  in  this  arch  will  be  formed  by  conical  surfaces 
passing  through  the  inner  edges  of  the  stones  which  correspond 
to  the  string  courses  ;  and  the  other  series  will  be  planes  passed 
through  the  axis  about  which  the  semi-circle  is  revolved.  This 
last  series  should  break  joints  with  each  other. 

455.  The  soffit  of  a  dome  is  usually  formed  by  revolving  the 
quadrant  of  one  of  the  usual  curves  of  cylindrical  arches  around 
the  rise  of  the  curve  ;  or  else  by  revolving  the  semi-curve  about 
the  line  of  the  span,  and  taking  the  half  of  the  surface  thus  gen 
erated  for  the  soffit  of  the  dome.  In  the  first  of  these  cases  the 
hori^pntal  section  of  the  dome  at  the  springing  line  will  be  a  cir- 
cle ;  in  the  second  the  entire  curve  of  the  semi-curve  by  which 
the  soffit  is  generated.  The  plan  of  domes  may  also  be  of  regu- 
lar polygonal  figure?  ,  in  which  case  the  soffit  will  be  a  polygonal- 


CONSTRUCT  :ON  OF  MASONRY. 


155 


cloistered  arch  formed  of  equal  sections  of  cylinders,  (Fig.  45  ) 
The  joints  and  the  bond  are  determined  in  the  same  manner  ai 
in  other  arches." 


Fig.  45— Represents  a  section  M  of  the  voussoirs  and 
an  elevation  of  the  soif  it ,  with  a  plan  N  of  the  soffit 
of  an  octagonal-cloistered  dome. 

The  letters  refer  to  the  same  parts  as  in  Fig.  43. 




J      1 

1    \     I     1 

1  \\  \  1 

:  o  \ \\jJ-U 

i  °^y^ 

456.  The  voussoirs  which  form  the  ring  course  of  the  heads, 
in  ordinary  cylindrical  arches,  are  usually  terminated  by  plane 
surfaces  at  top  and  on  the  sides,  for  the  purpose  of  connecting 
them  with  the  horizontal  courses  of  the  head  which  lie  above  and 
on  each  side  of  the  arch,  (Figs.  46  and  47.)     This  connection 


Fig.  40— Represents  a  manner  of  connecting  the  voussoirs 

and  horizontal  courses  in  an  oval  arch. 
o,  o,  are  examples  of  voussoirs  with  elbow  joints. 


Fig.  47— Represents  a  mode  of  arranging  the  voussoira 
and  horizontal  courses  in  flat  segment  arches 


may  be  arranged  in  a  variety  of  ways.  The  two  points  to  be 
kept  in  view  are,  to  form  a  good  bond  between  the  voussoirs  and 
horizontal  courses,  and  to  give  a  pleasing  architectural  effect  by 
the  arrangement.  This  connection  should  always  give  a  sym- 
metrical appearance  to  the  halves  of  the  structure  on  each  side 
of  the  crown.  To  effect  these  several  objects  it  may  be  neces- 
sary, in  cases  of  oval  arches,  to  make  the  breadth  of  the  voussoirs 
unequal,  diminishing  usually  those  near  the  springing  lines. 

457.  In  small  arches  the  voussoirs  near  the  springing  line  are 
so  cut  as  to  form  a  part  also  of  the  horizontal  course,  (see  Fig. 
46,)  forming  what  is  termed  an  elbow  joint.     This  plan  is  objec 


156  MASONRY. 

tionable,  both  because  there  is  a  waste  of  material  in  forming 
a  joint  of  this  kind,  and  the  stone  is  liable  to  crack  when  the 
arch  settles. 

458.  The  forms  and  dimensions  of  the  voussoirs  should  be  de- 
termined both  by  geometrical  drawings  and  numerical  calcula- 
tion, whenever  the  arch  is  important,  or  presents  any  complication 
of  form.  The  drawings  should,  in  the  first  place,  be  made  to  a 
scale  sufficiently  large  to  determine  the  parts  with  accuracy,  and 
from  these,  pattern  drawings  giving  the  parts  in  their  true  size 
may  be  made  for  the  use  of  the  mason.  To  make  the  pattern 
drawings,  the  side  of  a  vertical  wall,  or  a  firm  horizontal  area 
may  be  prepared,  with  a  thin  coating  of  mortar,  to  receive  a  thin 
smooth  coat  of  plaster  of  Paris.  The  drawing  may  be  made  on 
this  surface  in  the  usual  manner,  by  describing  the  curve  either 
by  points  from  its  calculated  abscissas  and  ordinates,  or,  where 
it  is  formed  of  circular  arcs,  by  using  the  ordinary  instrument  for 
describing  such  arcs  when  the  centres  fall  within  the  limits  of  the 
prepared  surface.  In  ovals  the  positions  of  the  extreme  radi; 
should  be  accurately  drawn  either  from  calculation,  or  construc- 
tion. To  construct  the  intermediate  normals,  whenever  the  cen- 
tres of  the  arcs  do  not  fall  on  the  surface,  an  arc  with  a  chord  of 
about  one  foot,  may  be  set  off  on  each  side  of  the  point  through 
which  the  normal  is  to  be  drawn,  and  the  chord  of  the  whole  arc, 
thus  set  off,  be  bisected  by  a  perpendicular.  This  construction 
will  generally  give  a  sufficiently  accurate  practical  result,  for 
elliptical  and  other  curves  of  a  large  size. 

459.  The  masonry  of  arches  may  be  either  of  dressed  stone, 
rubble,  or  brick. 

In  wide  spans,  particularly  for  oval  and  other  flat  arches,  cut 
stone  should  alone  be  used.  The  joints  should  be  dressed  with 
extreme  accuracy.  As  the  voussoirs  have  to  be  supported  by  a 
framing  of  timber,  termed  a  centre,  until  the  arch  is  completed, 
and  as  this  structure  is  liable  to  yield,  both  from  the  elasticity  p£ 
the  materials  and  the  number  of  joints  in, the  frame,  an  allowance 
for  the  settling  in  the  arch,  arising  from  these  causes,  is  some- 
times made,  in  cutting  the  joints  of  the  voussoirs  false,  that  is, 
not  according  to  the  true  position  of  the  normal,  but  from  >he 
supposed  position  the  joints  will  take  when  the  arch  has  settled 
thoroughly.  The  object  of  this  is  to  bring  the  surfaces  of  the 
joints  into  perfect  contact  when  the  arch  has  assumed  its  perma- 
nent state  of  equilibrium,  and  thus  prevent  the  voussoirs  from 
breaking  by  unequal  pressures  on  their  coursing  joints.  This 
is  a  problem  of  considerable  difficulty,  and  it  will  generally  be 
better  to  cut  the  joints  true,  and  guard  against  settling  and  its 
effects  by  giving  great  stiffness  to  the  centres,  and  by  placing  be- 
tween the  joints  of  those  voussoirs,  where  the  principal  movement 


CONSTRUCTION  OF  MASONRY. 


157 


lakes  place  in  arches,  sheets  of  lead  suitably  hammered  to  fit  the 
joint  and  yield  to  any  pressure. 

460.  The  manner  of  laying  the  voussoirs  demands  peculiar 
care,  particularly  in  those  which  form  the  heads  of  the  arch. 
The  positions  of  the  inner  t  dges  of  the  voussoirs  are  determined 
by  fixed  lines,  marked  on  the  abutments,  or  some  other  immovea- 
ble object,  and  the  calculated  distances  of  the  edges  from  these 
Jines.  These  distances  can  be  readily  set  off  by  means  of  the 
level  and  plumb-line.  The  angle  of  each  joint  can  be  fixed  by  a 
quadrant  of  a  circle,  connected  with  a  plumb-line,  on  which  the 
position  of  each  joint  is  marked. 

461.  Rubble  stone  is  used  only  for  very  small  arches  which 
do  not  sustain  much  weight,  or  as  a  filling  between  a  network  of 
ring  and  string  courses  in  large  arches  which  sustain  only  their 
own  weight.  In  each  case  the  blocks  of  rubble  should  be  roughly 
dressed  with  the  hammer,  and  be  laid  in  good  hydraulic  mortar. 

462.  Brick  may  be  used  alone,  or  in  combination  with  cut 
stone,  for  arches  of  considerable  size.  When  the  thickness  of  a 
brick  arch  exceeds  a  brick  and  a  half,  the  bond  from  ihe  soffit 
outward  presents  some  difficulties.  If  the  bricks  are  laid  in  con- 
centric layers,  or  shells,  a  continuous  joint  will  be  formed  parallel 
to  the  surface  of  the  soffit,  which  will  probably  yield  when  the 
arch  settles,  causing  the  shells  to  separate,  (Fig.  48.)     If  the 


Fig.  48— Represent*  wt  end 
view  M  of  a  bricx  arch 
built  with  blocks  C.  and 
shells  A,  and  B. 

N,  represents  the  manner 
of  arranging  the  courses 
of  brick  forming  the 
crown  of  the  arch. 


bricks  are  laid  like  ordinary  string  courses,  forming  continuous 
joints  from  the  soffit  outward,  these  joints,  from  the  form  of  the 
bricks,  will  be  very  open  at  the  back,  and,  from  the  yielding  of 
the  mortar,  the  arch  will  be  liable  to  injury  in  settling  from  this 
cause.  To  obviate  both  of  these  defects,  the  arch  may  be  built 
partly  by  the  first  plan  and  partly  by  the  second,  or  as  it  is  termed, 
in  shells  and  blocks.  The  crown,  or  key  of  the  arch  should  be 
laid  in  a  block,  increasing  the  breadth  of  the  block  by  two  bricks 
for  each  course  from  the  soffit  outward.     These  bricks  should  be 


158 


MASONRY. 


iaid  in  hydraulic  cement,  and  be  well  wedged  with  pieces  of  thin 
hard  slate  between  the  joints. 

463.  When  a  combination  of  brick  and  cut  stone  is  used,  the 
ring  courses  of  the  heads,  with  some  intermediate  ring  courses, 
the  bottom  string  courses,  the  key-stone  course,  and  a  few  inter- 
mediate string  courses,  are  made  of  cut  stone,  (Fig.  49,)  the 


Fig.  49  —  Repre- 
sents a  cross  sec- 
tion of  a  stone 
segment  arch 
capped  with 
brick  and  beton. 

A,  stone  voussoirs. 

B  and  D,  brick  and 
beton  capping. 

C,  abutment. 

E,  cushion  stone. 


intermediate  spaces  being  filled  in  with  brick.     The  brick  poa 
tions  of  the  soffit  may,  if  necessary,  be  thrown  within  the  stone 
portions,  forming  plain  caissons. 

464.  The  centres  of  small  arches  are  not  removed,  or  struck 
until  the  mortar  has  become  hard ;  in  large  arches,  the  centres 
should  not  be  struck  until  the  whole  of  the  mortar  has  set  firmly. 
In  the  joints  near  the  springing  lines  the  mortar  will  have  become 
hard,  in  the  ordinary  progress  of  building  an  arch,  before  that  in 
the  higher  joints  will  have  had  time  to  set,  unless  hydraulic  mor- 
tar of  a  quick  set  be  used.  After  the  centres  are  struck,  the  arch 
is  allowed  to  assume  its  permanent  state  of  equilibrium,  before 
any  of  the  superstructure  is  laid. 

465.  When  the  heads  of  the  arch  form  a  part  of  an  exterior 
surface,  as  the  faces  of  a  wall,  or  the  outer  portions  of  a  bridge, 
the  voussoirs  of  the  head  ring  courses  are  connected  with  the 
horizontal  courses,  as  has  been  explained  ;  the  top  surface  of  the 
voussoirs  of  the  intermediate  ring  courses  are  usually  left  in  a 
roughly  dressed  state  to  receive  the  courses  of  masonry  termed 
the  capping,  (see  Fig.  49,)  which  rests  upon  the  arch  between 
the  walls  of  the  head.  Before  laying  the  capping,  the  joints  of 
the  voussoirs  on  the  back  of  the  arch  should  be  carefully  exam- 
ined, and,  wherever  they  are  found  to  be  open  from  the  settling 
of  the  arch,  they  should  be  filled  up  with  soft-tempered  mortar, 
and  by  driving  in  pieces  of  hard  slate.  The  capping  may  be  va- 
riously formed  of  rubble,  brick,  or  beton.  Where  the  arches  are 
exposed  to  the  filtration  of  rain  water,  as  in  those  used  for  bridges, 
and  the  casemates  of  fortifications,  the  capping  should  be  of  beton 


CONSTRUCTION  OF  MASONRY.  159 

(aid  in  layers,  and  well  rammed  with  the  usual  precautions  for 
obtaining  a  solid  homogeneous  mass. 

466.  The  difficulty  of  forming  water-tight  cappings  of  mason- 
ry has  led  engineers,  within  a  few  years  back,  to  try  a  coating  of 
asphalte  upon  the  surface  of  beton.  The  surface  of  the  beton 
capping  is  made  uniform  and  smooth  by  the  trowel,  or  float,  and 
the  mass  is  allowed  to  become  thoroughly  :'jy  before  the  asphalte 
is  laid.  Asphalte  is  usually  laid  on  in  twc  layers.  Before  apply- 
ing the  first,  the  surface  of  the  beton  should  be  thoroughly 
cleansed  of  dust,  and  receive  a  coating  of  mineral  tar  applied  hot 
with  a  swab.  This  application  of  hot  mineral  tar  is  said  to  pre- 
vent the  formation  of  air  bubbles  in  the  layers  of  asphalte  which, 
when  present,  permit  the  water  to  percolate  through  the  masonry. 
The  first  layer  of  asphalte  is  laid  on  in  squares,  or  thin  blocks, 
care  being  taken  to  form  a  perfect  union  between  the  edges  of 
the  squares  by  pouring  the  hot  liquid  along  them  in  forming  each 
new  one.  The  surface  of  the  first  layer  is  made  uniform,  and 
rubbed  until  it  becomes  smooth  and  hard  with  an  ordinary  wooden 
float.  In  laying  the  second  layer,  the  same  precautions  are  taken 
as  for  the  first,  the  squares  breaking  joints  with  those  of  the  first. 
Fine  sand  is  strewed  over  the  surface  of  the  top  layer,  and  pressed 
into  the  asphalte  before  it  becomes  hard. 

Coverings  of  asphalte  have  been  used  both  in  Europe  and  in 
our  military  structures  for  some  years  back  with  decided  success. 
There  have  been  failures,  in  some  instances,  arising  in  all  prob- 
ability either  from  using  a  bad  material,  or  from  some  fault  of 
workmanship. 

467.  In  a  range  of  arches,  like  those  of  bridges,  or  casemates, 
the  capping  of  each  arch  is  shaped  with  two  inclined  surfaces, 
like  a  common  roof.  The  bottom  of  these  surfaces,  by  their 
junction,  form  gutters  where  the  water  collects,  and  from  which 
it  is  conveyed  off  in  conduits,  formed  either  of  iron  pipes,  or  of 
vertical  openings  made  through  the  masonry  of  the  piers  which 
communicate  with  horizontal  covered  drains.  A  small  arch  of 
sufficient  width  to  admit  a  man  to  examine  its  interior,  or  a  square 
culvert,  is  formed  over  the  gutter.  When  the  spaces  between  the 
head  walls  above  the  capping  is  filled  in  with  earth,  a  series  of 
drains  running  from  the  top,  or  ridge  of  the  capping,  and  leading 
into  the  main  gutter  drain,  should  be  formed  of  brick.  They 
may  be  best  made  by  using  dry  brick  laid  flat,  and  with  intervals 
left  for  the  drains,  these  being  covered  by  other  courses  of  dry 
brick  with  the  joints  in  some  degree  open.  The  earth  is  filled  in 
upon  the  upper  course  of  bricks,  which  should  be  so  laid  as  to 
form  a  uniform  surface. 

468.  When  the  space  above  the  capping  is  not  filled  in  with  a 
L'olid  mass,  for  the  purpose  of  receiving  the  weight  borne  by  the 


160 


MASONRY. 


arches,  walls  of  a  requisite  height  may  be  built  parallel  to  the 
head  walls,  and  these  may  serve  either  as  the  piers  of  small 
arches,  (Fig.  50,)  upon  which  the  weight  borne  directly  rests,  or 


Fig.  50 — Represents  a  section  through 
a  pier  and  the  heads  of  an  arch, 
showing  the  manner  in  which  small 
arches  are  built  on  piers  C,  C,  paral- 
lel to  the  head  walls  B,  to  sustain  the 
load  above  the  arch 


else  be  covered  by  strong  flat  stones  to  effect  the  same  object 
In  this  last  case  (Fig.  51 )  the  walls  may  be  made  lighter  by  form 


wmwm//M/mM///////m/wmvm/MMM 


Fig.  51— Represents  a  cross  section 
of  the  parts  of  two  arches,  and  the 
pier  A,  showing  the  manner  in 
which  walls  B,  with  arched  open- 
ings C.  C  through  them  are  built 
parallel  to  the  heads,  to  receive  the 
flat  stones  a,  a  which  support  the 
load  above  the  arches. 


ine  arched  openings  through  them,  or  else  a  system  of  small  right 
cylindrical  groined  arches  may  be  used.  All  of  these  methods  are 
in  use  in  bridge  building  for  sustaining  the  roadway,  and  also  in 
roofing  arched  edifices.  They  throw  less  weight  upon  the  abut- 
ments and  piers  of  the  arches  than  would  a  filling  of  solid  ma- 
terial. 

469.  From  observations  taken  on  the  manner  in  which  large 
cylindrical  arches  settle,  and  experiments  made  on  a  small  scale, 
it  appears  that  in  all  cases  of  arches  where  the  rise  is  equal  to 
or  less  than  the  half  span  they  yield  (Fig.  52)  by  the  crown  of 
the  arch  falling  inward,  and  thrusting  outward  the  lower  portions, 
presenting  five  joints  of  rupture,  one  at  the  key  stone,  one  on  each 
side  of  it  which  limit  the  portions  that  fall  inward,  and  one  on 
each  side  near  the  springing  lines  which  limit  the  parts  thrust 


CONSTRUCTION  OF  MASONRY.  161 

outward .     In  pointed  arches,  or  those  in  which  the  rise  is  greater 

j^y    m  m    n£\  Fie.  52 — Represents  the  manner  in  which  flat  arches  yield 

A/  \A  by  rupture 

Ay  Vjy        o,  joint  of  rupture  at  the  key  stone. 

O  V-\        ?n,  ?/i,  joints  of  rupture  below  the  key  stone. 

r~  n    n,  n.  joints  of  rupture  at  springing  lines. 

than  the  half  span,  the  tendency  to  yielding  is,  in  some  cases, 
different ;  here  the  lower  parts  may  fall  inward,  (Fig.  53,)  and 
thrust  upward  and  outward  the  parts  near  the  crown. 


Fig.  53 — Represents  the  manner  in  which  pointed  arches 

may  yield. 
The  letters  refer  to  same  points  as  in  Fig.  52. 


470.  From  this  movement  in  arches  a  pressure  arises  against 
the  key  stone,  termed  the  horizontal  thrust  of  the  arch,  the 
tendency  of  which  is  to  crush  the  stone  at  the  key,  and  to  over- 
turn the  abutments  of  the  arch,  causing  them  to  rotate  about 
the  exterior  edge  of  some  one  of  their  horizontal  joints. 

471.  The  joints  of  rupture  below  the  key  stone  vary  in  arch- 
es of  different  forms,  and  in  the  same  arch  with  the  weight  it  sus- 
tains. From  experiments,  it  appears  that  in  full  centre  arches 
the  joints  in  question  make  an  angle  of  about  27°  with  the 
horizon  ;  in  segment  arches  of  arcs  less  than  120°  they  are  at 
the  springing  lines  ;  and  in  oval  arches  of  three  centres  they  are 
found  about  the  angle  of  45°  of  the  small  arc  which  forms  the 
extremity  of  the  curve  at  the  springing  line. 

472.  'the  calculation  of  the  joints  of  rupture,  the  consequent 
horizontal  thrust,  and  its  effects  in  crushing  the  stone  at  the 
key  and  in  overturning  the  abutment  are  problems  of  conside- 
rable mathematical  intricacy.  When  the  joints  of  rupture  are 
given  the  problem  assumes  a  more  simple  form,  being  one  of 
statical  equilibrium  between  the  moments  of  the  horizontal 
thrust  and  the  weight  of  the  arch  and  its  abutments. 

The  problem  for  finding  the  joints  of  rupture  by  calculation, 
and  the  consequent  thickness  of  the  abutments  necessary  to 
preserve  the  arch  from  yielding,  has  been  solved  by  a  number 
of  writers  on  the  theory  of  the  equilibrium  of  arches,  and  tables 
for  effecting  the  necessary  numerical  calculations  have  been 
drawn  up  from  their  results  to  abridge  the  labor  in  each  case. 

473.  The  connection  between  the  top  of  the  abutment,  term- 
ed the  impost  of  the  arch,  and  the  bottom  courses  of  the  arch, 

21 


162  MASONRY. 

requires  peculiar  care  in  segmental,  askew,  and  rampant  arches. 
In  the  first,  the  thrust  of  the  arch  being  very  great,  it  will  be 
well,  in  heavy  arches,  to  make  the  joints  of  the  interior  courses 
of  the  abutment,  for  some  courses  at  least  below  the  impost,  ob- 
lique to  the  horizon  to  counteract  any  danger  from  sliding.  The 
top  stone  of  the  abutment,  termed  the  cushion  stone  of  the  arch, 
should  be  well  bonded  with  the  stones  of  the  backing,  and  its  bed, 
or  bottom  joint  should  be  so  far  below  the  impost  joint,  that  the 
stone  shall  offer  sufficient  strength  to  resist  the  pressure  on  it. 

In  the  askew  arch  the  abutments  are  not  uniformly  loaded, 
and  the  entire  thrust  of  the  arch  will  not  be  received  by  the 
abutments  if  the  arch  is  constructed  in  the  usual  manner.  Each 
of  these  points  requires  peculiar  attention  ;  the  first  demanding 
the  thickness  of  the  abutment  to  be  suitably  regulated ;  the  se- 
cond that  the  arch  be  so  built  that  the  thrust  may  be  thrown,  as 
nearly  as  practicable,  parallel  to  the  planes  of  the  heads.  To 
effect  this  last  point,  the  portion  of  the  arch  above  the  upper 
joints  of  rupture  (Fig.  54,)  must  be  divided  into  several  zones,  each 
of  these  zones  being  built  without  any  connection  with  the  two 
adjacent  to  it,  but  with  their  ends  so  arranged  that  this  connec- 
tion may  be  formed,  and  the  arch  made  continuous  after  the 


Fig.  54 — Represents  the  development  of  half  of  the 
soffit  of  an  oblique  cylindrical  arch  with  helicoidal 
joints,  showing  the  divisions  of  the  soffit  into  zones 
A,  B,  C,  D  by  a  series  of  heading  joints  mn  laid  open 
without  mortar. 

acb,  development  of  curve  of  oblique  section. 

ce,  one  of  the  edges  of  the  coursing  joints  perpendicu- 
lar to  the  right  line  ab. 

ad,  springing  line  of  arch. 


centres  are  struck.  By  this  plan  the  settling  will  take  place 
after  uncentring  without  causing  cracks,  and  the  thrust  will  be 
thrown  on  the  abutments  in  the  direction  desired. 

In  rampant  arches,  the  impost  joint  being  oblique  to  the  ho- 
rizon, care  must  be  taken,  if  this  obliquity  be  not  less  than  the 
angle  of  friction  of  the  stone  used,  either  to  cut  the  impost  into 
steps,  or  else  to  use  some  suitable  bond,  or  iron  cramps  and 
bolts  to  prevent  disjunction  between  the  arch  and  abutment. 

474.  The  abutments  of  right  and  of  slightly  oblique  cylindri- 
cal arches  are  made  of  uniform  dimensions  ;  but  when  the  ob  • 


CONSTRUCTION'  OF  MASONRY.  16$ 

Iiquity  is  considerable,  it  may  be  necessary  to  increase  the  thick- 
ness of  a  portion  of  each  abutment  where  there  is  the  greatest 
pressure. 

In  conical  and  conoidal  arches  the  abutments  will  in  like  man 
ner  vary  in  dimensions  with  the  span. 

475.  In  cloistered  arches  the  abutments  will  be  less  than  in  an 
ordinary  cylindrical  arch  of  the  same  length ;  and  in  groined 
arches,  in  calculating  the  resistance  offered  by  the  abutments, 
tlie  counter  resistance  offered  by  the  weight  of  one  portion  in 
resisting  the  thrust  of  the  other,  must  be  taken  into  consideration. 

476.  When  abutments,  as  in  the  case  of  edifices,  require  to  be 
of  considerable  height,  and  therefore  would  demand  extraordinary 
thickness,  if  used  alone  to  sustain  the  thrust  of  the  arch,  they  mav 
be  strengthened  by  the  addition  to  their  weight  made  in  carrying 
them  up  above  the  imposts  like  the  battlements  and  pinnacles  in 
Gothic  architecture  ;  by  adding  to  them  ordinary,  full,  or  arched 
buttresses,  termed  flying  buttresses ;  or  by  using  ties  of  iron  con- 
necting the  voussoirs  near  the  joints  of  rupture  below  the  kev 
stone.  The  employment  of  these  different  expedients,  their  forms 
and  dimensions,  will  depend  on  the  character  of  the  structure 
and  the  kind  of  arch.  The  iron  tie,  for  example,  cannot  be  hid- 
den from  view  except  in  the  plate-bande,  or  in  very  flat  segment 
arches,  and  wherever  its  appearance  would  be  unsightly  some 
other  expedient  must  be  tried. 

Circular  rings  of  iron  have  been  used  to  strengthen  the  abut- 
ments of  domes,  by  confining  the  lower  courses  of  the  dome  and 
relieving  the  abutment  from  the  thrust. 

477.  When  abutments  sustain  several  arches  above  each  other, 
like  relieving  arches  in  tiers,  their  dimensions  must  be  calculated 
to  sustain  the  united  thrusts  of  the  arches ;  and  the  several  por- 
tions between  each  tier  must  be  strong  enough  to  resist  the  thrust 
of  their  corresponding  arches. 

478.  In  a  range  of  arches  of  unequal  size,  the  piers  will  have 
to  sustain  a  lateral  pressure  occasioned  by  the  unequal  horizontal 
thrust  of  the  arches.  In  arranging  the  form  and  dimensions  of 
the  piers  this  inequality  of  thrust  must  be  estimated  for,  taking 
also  into  consideration  the  position  of  the  imposts  of  the  unequal 
arches. 

479.  Precautions  against  Settling.  One  of  the  most  difficult 
and  important  problems  in  the  construction  of  masonry,  is  that 
of  preventing  unequal  settling  in  parts  which  require  to  be  con- 
nected but  sustain  unequal  weights,  and  the  consequent  ruptures 
in  the  masses  arising  from  this  cause.  To  obviate  this  difficulty 
requires  on  the  part  of  the  engineer  no  small  degree  of  practical 
tact.  Several  precautions  must  be  taken  to  diminish  as  far  as 
practicable  the  danger  from  unequal  settling.     Walls  sustaining 


164  MASONS  Y. 

heavy  vertical  pressu.es  should  be  built  up  uniformly,  ard  with 
great  attention  to  the  bond  and  correct  titling  of  the  courses.  The 
materials  should  be  uniform  in  quality  and  size ;  hydraulic  mor- 
tar should  alone  be  used ;  and  the  permanent  weight  not  be  laid 
en  the  wall  until  the  season  after  the  masonrv  is  laid.  As  a  far- 
ther precaution,  when  practicable,  a  trial  weight  may  be  laid  upon 
the  wall  before  loading  it  with  the  permanent  one. 

Where  the  heads  of  arches  are  built  into  a  wall,  particularly 
if  they  are  designed  to  bear  a  heavy  permanent  weight,  as  an 
embankment  of  earth,  the  wall  should  not  be  carrried  up  higher 
than  the  imposts  of  the  arches  until  the  settling  of  the  latter  has 
reached  its  final  term  ;  and  as  there  will  be  danger  of  disjunction 
between  the  piers  of  the  arches  and  the  wall  at  the  head,  from 
the  same  cause,  these  should  be  carried  up  independently,  but  sc 
arranged  that  their  after-union  may  be  conveniently  effected.  It 
would  moreover  be  always  well  to  suspend  the  building  of  the 
arches  until  the  season  following  that  in  which  the  piers  are 
finished,  and  not  to  place  the  permanent  weight  upon  the  arches 
until  the  season  following  their  completion. 

480.  Pointing.  The  mortar  in  the  joints  near  the  surfaces  of 
walls  exposed  to  the  weather  should  be  of  the  best  hydraulic 
lime,  or  cement,  and  as  this  part  of  the  joint  always  requires  to 
be  carefully  attended  to,  it  is  usually  filled,  or  as  it  is  termed 
pointed,  some  time  after  the  other  work  is  finished.  The  period 
at  which  pointing  should  be  done  is  a  disputed  subject  among 
builders,  some  preferring  to  point  while  the  mortar  in  the  joint  is> 
still  fresh,  or  green,  and  others  not  until  it  has  become  hard. 
The  latter  is  the  more  usual  and  better  plan.  The  mortar  for 
pointing  should  be  poor,  that  is,  have  rather  an  excess  of  sand ; 
the  sand  should  be  of  a  fine  uniform  grain,  and  but  little  water 
be  used  in  tempering  the  mortar.  Before  applying  the  pointing, 
the  joint  should  be  well  cleansed  by  scraping  and  brushing  out 
the  loose  matter,  and  then  be  well  moistened.  The  mortar  is 
applied  with  a  suitable  tool  for  pressing  it  into  the  joint,  and  its 
surface  is  rubbed  smooth  with  an  iron  tool.  The  practice  among 
our  military  engineers  is  to  use  the  ordinary  tools  for  calking  in 
applying  pointing ;  tp  calk  the  joint  with  the  mortar  in  the  usual 
way,  and  to  rub  the  surface  of  the  pointing  until  it  becomes  hard. 
To  obtain  pointing  that  will  withstand  the  vicissitudes  of  our  cli- 
mate is  not  the  least  of  the  difficulties  of  the  builder's  art.  The 
contraction  and  expansion  of  the  stone  either  causes  the  pointing 
to  crack,  or  else  to  separate  from  the  stone,  and  the  surface  water 
penetrating  into  the  cracks  thus  made,  when  acted  upon  by  frost 
throws  out  the  pointing.  Some  have  tried  to  meet  this  difficulty 
by  giving  the  surface  of  the  pointing  such  a  shape,  and  so  ar 
ranging  it  with  respect  to  the  surfaces  of  the  stones  forming  the 


CONSTRUCTION  OF  MASONRY.  165 

pint,  that  the  water  shall  trickle  over  the  pointing  without  enter- 
ing the  crack  which  is  usually  between  the  bed  of  the  stone  and 
the  pointing. 

481.  The  term  flash  pointing  is  sometimes  applied  to  a  coat- 
ing of  hydraulic  mortar  laid  over  the  face,  or  back  of  a  wall,  tc 
preserve  either  the  mortar  joints,  or  the  stone  itself  from  the  action 
of  moisture,  or  the  effects  of  the  atmosphere.  Mortar  for  flash 
pointing  should  also  be  made  poor,  and  when  it  is  used  as  a  stucco 
to  protect  masonry  from  atmospheric  action,  it  should  be  made  of 
coarse  sand,  and  be  applied  in  a  single  uniform  coat  over  the  sur- 
face, which  should  be  prepared  to  receive  the  stucco  by  having 
the  joints  thoroughly  cleansed  from  dust  and  loose  mortar,  and 
being  well  moistened. 

No  pointing  of  mortar  has  been  found  to  withstand  the  effects 
of  weather  in  our  climate  on  a  long  line  of  coping.  Within  a  few 
years  a  pointing  of  asphalte  has  been  tried  on  some  of  our  mili- 
tary works,  and  has  given  thus  far  promise  of  a  successful  issue. 

482.  Stucco  exposed  to  weather  is  sometimes  covered  with 
paint,  or  other  mixtures,  to  give  it  durability.  Coal  tar  has  been 
tried,  but  without  success  in  our  climate.  M.  Raucourt  de 
Charleville,  in  his  work  Traite  des  Mortiers,  gives  the  following 
compositions  for  protecting  exposed  stuccoes,  which  he  states  to 
succeed  well  in  all  climates.  For  important  work,  three  parts  of 
linseed  oil  boiled  with  one  sixth  of  its  weight  of  litharge,  and  one 
part  of  wax.  For  common  works,  one  part  of  linseed  oil,  one 
tenth  of  its  weight  of  litharge,  and  two  or  three  parts  of  resin. 

The  surfaces  must  be  thoroughly  dry  before  applying  the 
compositions,  which  should  be  laid  on  hot  with  a  brush. 

483.  Repairs  of  Masonry.  In  effecting  repairs  in  masonry, 
when  new  work  is  to  be  connected  with  old,  the  mortar  of  the  old 
should  be  thoroughly  cleaned  off  wherever  it  is  injured  along  the 
surface  where  the  junction  is  effected.  The  bond  and  other  ar- 
rangements will  depend  upon  the  circumstances  of  the  case  ;  the 
surfaces  connected  should  be  fitted  as  accurately  as  practicable, 
so  that  by  using  but  little  mortar,  no  disunion  may  take  place 
from  settling. 

484.  An  expedient,  very  fertile  in  its  applications  to  hydraulic 
constructions,  has  been  for  some  years  in  use  among  the  French 
engineers,  for  stopping  leaks  in  walls  and  renewing  the  beds  of 
foundations  which  have  yielded,  or  have  been  otherwise  removed 
by  the  action  of  water.  It  consists  in  injecting  hydraulic  cement 
into  the  parts  to  be  filled,  through  holes  drilled  through  the  ma- 
sonry, by  means  of  a  strong  syringe.  The  instruments  used  for 
this  purpose  (Fig.  55)  are  usually  cylinders  of  wood,  or  of  cast 
iron  ;  the  bore  uniform,  except  at  the  end  which  is  terminated 
with  a  nozle  ( f  the  usual  conical  form  ;  the  piston  is  of  wood 


166 


MASONRY 


and  is  dnven  down  by  a  heavy  mallet.     In   ising  the  aynnge,  ii 
is  adjusted  to  the  hole ;  the  hydraulic  cement  in  a  semi-flu  it 


Fig.  55 — Represents  the  arrangements  for  in 
jeoting  hydraulic  cement  under  a  wall. 

A,  section  of  the  wall  with  vertical  holes  c,  e 
drilled  through  it. 

B,  syringe  and  piston  for  injecting  the  cement 
into  the  space  C  under  the  wall. 


state  poured  into  it ;  a  wad  of  tow,  or  a  disk  of  leather  being  in- 
troduced on  top  before  inserting  the  piston.  The  cement  is 
forced  in  by  repeated  blows  on  the  piston. 

485.  A  mortar  of  hydraulic  lime  and  fine  sand  has  been  used 
for  the  same  purpose  ;  the  lime  being  ground  fresh  from  the  kiln, 
and  used  before  slaking,  in  order  that  by  the  increase  of  volume 
which  takes  place  from  slaking,  it  might  fill  more  compactly  all 
interior  voids.  The  use  of  unslaked  lime  has  received  several 
ingenious  applications  of  this  character ;  its  after  expansion  may 
prove  injurious  when  confined.  The  use  of  sand  in  mortar  for 
injections  has  by  some  engineers  been  condemned,  as  from  the 
«tate  of  fluidity  in  which  the  mortar  must  be  used,  it  settles  to 
the  bottom  of  the  syringe,  and  thus  prevents  the  formation  of  a 
homogeneous  mass. 

486.  Effects  of  Temperature  on  Masonry.  Frost  is  the  most 
powerful  destructive  agent  against  which  the  engineer  has  to 
guard  in  constructions  oi  masonry.  During  severe  winters  in  the 
northern  parts  of  our  country,  it  has  been  ascertained,  by  obser- 
vation, that  the  frost  will  penetrate  earth  in  contact  with  walls  to 
depths  exceeding  ten  feet ;  it  therefore  becomes  a  matter  of  the 
first  importance  to  use  every  practicable  means  to  drain  thoroughly 
all  the  ground  in  contact  with  masonry,  to  whatever  depths  the 
foundations  may  be  sunk  below  the  surface  ;  for  if  this  precau- 
tion be  not  taken,  accidents  of  the  most  serious  nature  may  hap- 
pen to  the  foundations  from  the  action  of  the  frost.  If  watei 
collects  in  any  quantity  in  the  earth  around  the  foundations,  it 


CONSTRUCTION  OF  MASONRY.  167 

may  be  necessary  to  make  small  covered  drains  under  them  to 
convey  it  off,  and  to  place  a  stratum  of  loose  stone  between  the 
sides  of  the  foundations  and  the  surrounding  earth  to  give  it  a 
free  downward  passage. 

Tt  may  be  laid  down  as  a  maxim  in  building,  that  mortar  which 
18  exposed  to  the  action  of  frost  before  it  has  set,  will  be  so  much 
damaged  as  to  impair  entirely  its  properties.  This  fact  places  in 
a  stronger  light  what  has  already  been  remarked,  on  the  necessity 
of  laying  the  foundations  and  the  structure  resting  on  them  in  hy- 
draulic mortar,  to  a  height  of  at  least  three  feet  above  the  ground  ; 
for,  although  the  mortar  of  the  foundations  might  be  protected 
from  the  action  of  the  frost  by  the  earth  around  them,  the  parts 
immediately  above  would  be  exposed  to  it,  and  as  those  parts  at- 
tract the  moisture  from  the  ground,  the  mortar,  if  of  common 
lime,  would  not  set  in  time  to  prevent  the  action  of  the  frosts  of 
winter. 

In  heavy  walls  the  mortar  in  the  interior  will  usually  be  se- 
emed from  the  action  of  the  frost,  and  masonry  of  this  character 
might  be  carried  on  until  freezing  weather  commences ;  but  still 
in  all  important  works  it  will  be  by  far  the  safer  course  to  sus- 
pend the  construction  of  masonry  several  weeks  before  the  or- 
dinary period  of  frost. 

During  the  heats  of  summer,  the  mortar  is  injured  by  a  too 
rapid  drying.  To  prevent  this  the  stone,  or  brick,  should  be 
thoroughly  moistened  before  being  laid  ;  and  afterwards,  if  the 
weather  is  very  hot,  the  masonry  should  be  kept  wet  until  the 
mortar  gives  indications  of  setting.  The  top  course  should  al- 
ways be  well  moistened  by  the  workmen  on  quitting  their  work 
for  any  short  period  during  very  warm  weather. 

The  effects  produced  by  a  high  or  low  temperature  on  mortar 
in  a  green  state  are  similar.  In  the  one  case  the  freezing  of  the 
water  prevents  a  union  between  the  particles  of  the  lime  and 
sand ;  and  in  the  other  the  same  arises  from  the  water  being 
rapidly  evaporated.  In  both  cases  the  mortar  when  it  has  set  is 
weak  and  pulverulent. 


168  FRAMING 


FRAMING. 

487.  Framing  is  the  art  of  arranging  beams  of  solid  material* 
for  the  various  purposes  to  which  they  are  applied  in  structures. 
A  frame  is  any  arrangement  of  beams  made  for  sustaining  strains. 

488.  That  branch  of  framing  which  relates  to  the  combinations 
of  beams  of  timber  is  denominated  Carpentry. 

489.  Timber  and  iron  are  the  only  materials  in  common  use 
for  frames,  as  they  are  equally  suitable  to  resist  the  various 
strains  to  be  met  with  in  structures.  Iron,  independently  of 
offering  greater  resistance  to  strains  than  timber,  possesses  the 
farther  advantage  of  being  susceptible  of  receiving  the  most  suit- 
able forms  for  strength  without  injury  to  the  material ;  while  tim- 
ber, if  wrought  into  the  best  forms  for  the  object  in  view  may,  in 
some  cases,  be  greatly  injured  in  strength. 

490.  The  object  to  be  attained  in  framing  is  to  give,  by  a  suit- 
able combination  of  beams,  the  requisite  degree  of  strength  and 
stiffness  demanded  by  the  character  of  the  structure,  united  with 
a  lightness  and  an  economy  of  material  of  which  an  arrangement 
of  a  massive  kind  is  not  susceptible.  To  attain  this  end,  the 
beams  of  the  frame  must  be  of  such  forms,  and  be  so  combined 
that  they  shall  not  only  offer  the  greatest  resistance  to  the  efforts 
they  may  have  to  sustain,  but  shall  not  change  their  relative  po- 
sitions from  the  effect  of  these  efforts. 

491.  The  forms  of  the  beams  will  depend  upon  the  kind  of 
material  used,  and  the  nature  of  the  strain  to  which  it  may  be 
subjected,  whether  of  tension,  compression,  or  a  cross  strain. 

492.  The  general  shape  given  to  the  frame,  and  the  combina 
tions  of  the  beams  for  this  purpose,  will  depend  upon  the  objeci 
of  the  frame  and  the  directions  in  which  the  efforts  act  upon  it. 

In  frames  of  timber,  for  example,  the  cross  sections  of  eacn 
beam  are  generally  uniform  throughout,  these  sections  being 
either  circular,  or  rectangular,  as  these  are  the  only  simple  forms 
which  a  beam  can  receive  without  injury  to  its  strength.  In 
frames  of  cast  iron,  each  beam  may  be  cast  into  the  most  suitable 
form  for  the  strength  required,  and  the  economy  of  the  material. 

493.  In  combining  the  beams,  whatever  may  be  the  general 
shape  of  the  frame,  the  parts  which  compose  it  must,  as  far  as 
practicable,  present  triangular  figures,  each  side  of  the  triangles 
being  formed  of  a  single  beam ;  the  connection  of  the  beams  at 
the  angular  points,  termed  the  joints,  being  so  arranged  that  no 
yielding  can  take  place.    In  all  combinations,  therefore,  in  which 


FRAMING.  169 

the  principal  beams  form  polygonal  figures,  secondary  beams 
must  be  added,  either  in  the  directions  of  the  diagonals  of  the 
polygon,  or  so  as  to  connect  each  pair  of  beams  forming  an  angle 
of  the  polygon,  for  the  purpose  of  preventing  any  change  of  form 
of  the  figure,  and  of  giving  the  frame  the  requisite  stiffness. 
These  secondary  pieces  receive  the  general  appellation  of  braces. 
When  they  sustain  a  strain  of  compression  they  are  termed  struts; 
when  one  of  extension,  ties. 

494.  As  one  of  the  objects  of  a  frame  is  to  transmit  the  strain 
it  directly  receives  to  firm  points  of  support,  the  beams  of  which 
it  is  formed  should  be  so  combined  that  this  may  be  done  in  the 
way  which  shall  have  the  least  tendency  to  change  the  shape  of 
the  frame,  and  to  fracture  the  beams.  These  conditions  will 
be  best  satisfied  by  giving  the  principal  beams  of  the  frame  a 
position  such  that  the  strains  they  receive  shall  be  transmitted 
through  the  axes  of  the  beams  to  the  fixed  supports  ;  in  this  man- 
ner there  can  be  no  tendency  to  change  the  shape  of  the  frame,  ex- 
cept so  far  as  this  may  arise  from  the  contractions,  or  elongations 
of  the  beams,  caused  by  the  strains ;  and  as  all  unnecessary 
transversal  strains  will  in  like  manner  be  avoided,  the  resistances 
offered  by  the  beams  will  be  the  greatest  practicable. 

495.  Whenever  these  conditions  cannot  be  satisfied,  the  strains 
un  the  frame  should  be  so  combined  that  those  which  are  not 
transmitted  to  the  points  of  support  shall  balance,  or  destroy  each 
other ;  and  those  beams  which,  from  being  subjected  to  a  cross 
strain,  might  be  either  in  danger  of  rapture,  or  of  being  deflected 
to  so  great  a  degree  as  to  injure  the  stability  of  the  frame,  should 
be  supported  by  struts  abutting  either  against  fixed  supports,  or 
against  points  of  the  frame  where  the  pressure  thrown  upon  the 
strut  would  have  no  effect  in  changing  the  shape  of  the  frame. 

496.  The  points  of  support  of  a  frame  may  be  either  above,  or 
below  it.  Jn  the  first  case,  the  frame  will  consist  of  a  suspended 
system,  in  which  the  polygon  will  assume  a  position  of  stable 
equilibrium,  its  sides  being  subjected  to  a  strain  of  extension.  In 
the  second  case  the  frame,  if  of  a  polygonal  form,  must  satisfy 
the  essential  conditions  already  enumerated,  in  order  that  its  state 
of  equilibrium  shall  be  stable. 

497.  The  strength  of  the  frame  and  that  of  its  parts,  and  their 
consequent  dimensions,  must  be  regulated  by  the  strains  to  which 
they  are  subjected.  When  the  form  of  the  frame  and  the  direc- 
tion and  amount  of  the  strain  borne  by  it  are  given,  the  direction 
and  amount  of  the  strain  which  the  different  parts  sustain  can  be 
ascertained  by  the  ordinary  laws  of  statics,  and,  from  these  data, 
the  requisite  dimensions  and  forms  of  the  parts. 

498.  The  object  of  the  structure  will  necessarily  decide  the 
general  shape  of  the  frame,  as  well  as  the  direction  of  the  strains 

22 


170  FRAMING. 

to  which  it  will  be  subjected.  An  examination,  therefore,  of  the 
frames  adapted  to  some  of  the  more  usual  structures  will  be  the 
best  course  for  illustrating  both  the  preceding  general  principles 
and  the  more  ordinary  combinations  of  the  beams  and  joints. 

499.  Frames  of  Timber.  These  are  composed  either  entirely 
of  straight  beams,  or  of  a  combination  of  straight  beams  and  of 
arches  formed  by  bending  straight  beams. 

Pieces  of  crooked  timber  are  used  either  where  the  form  of  the 
parts  requires  them,  or  else  where  a  strong  connection  is  necessary 
between  straight  pieces  that  form  an  angle  between  them. 

500.  As  has  already  been  stated,  the  cross  section  of  each 
beam  is  generally  uniform  and  rectangular.  This  will,  in  some 
cases,  give  more  strength  than  the  character  of  the  strain  resisted 
may  demand  ;  and  will,  also,  throw  a  greater  amount  of  pressure 
on  the  points  of  support,  than  if  beams  of  a  form  more  strictly 
adapted  to  the  object  in  view  were  used :  but  it  avoids  cutting 
the  fibres  across  the  grain,  or  making,  as  it  is  termed,  grain-cm 
beams,  and  thereby  materially  injuring  the  strength  of  the  piece. 
This  objection,  however,  is  only  applicable  to  the  parts  of  a  frame 
formed  of  single  beams.  Wherever  several  thicknesses  of  beams 
are  required  in  the  arrangement  of  any  part,  the  advantage  may 
be  taken  of  giving  the  combination  the  most  suitable  form  for 
strength  and  lightness  combined. 

501.  Frames  for  Cross  Strains.  The  parts  of  a  frame  which 
receive  a  cross  strain  may  be  horizontal,  as  the  beams,  ox  joists  of 
a  floor;  or  inclined,  as  the  beams,  or  rafters  which  form  the  inclined 
sides  of  the  frame  of  a  roof.  The  pressure  producing  the  cross 
strain  may  either  be  uniformly  distributed  over  the  beams,  as  in 
the  cases  just  cited,  arising  from  the  flooring  boards  in  the  one 
case,  and  the  roof  covering  in  the  other ;  or  it  may  act  only  at  one 
point,  as  in  the  case  of  a  weight  laid  upon  the  beam. 

In  all  of  these  cases  the  extremities  of  the  beam  must  be  firmly 
fixed  against  immoveable  points  of  support ;  the  longer  side  of 
the  rectangular  section  of  the  beam  should  be  parallel  to  the  di- 
rection of  the  strain,  on  account  of  placing  the  beam  in  the  best 
position  for  strength. 

If  the  distance  between  the  points  of  support,  or  the  bearing, 
be  not  great,  the  framing  may  consist  simply  of  a  row  of  parallel 
beams  of  such  dimensions,  and  placed  so  far  asunder  as  the  strain 
borne  may  require.     When  the  beams  are  narrow,  or  the  depth. 


Fig.  5G— Represents  a  cross  section  of  horizontal  beams  a  c 
braced  by  diagonal  battens  b. 


of  the  rectangle  considerably  greater  than  the  breadth,  (Fig.  R#,J 


FRAMING. 


171 


chert  struts  of  battens  may  be  placed  at  intervals  betw  een  each 
pair  of  beams,  in  a  diagonal  direction,  uniting  the  bottom  of  the 
one  with  the  top  of  the  other,  to  prevent  the  beams  from  twisting, 
or  yielding  laterally. 

When  the  bearing  and  strain  are  so  great  that  a  single  beam 
will  not  present  sufficient  strength  and  stiffness,  a  combination 
of  beams,  termed  a  built  beam,  which  may  be  solid,  consisting 
of  several  layers  of  timber  laid  in  juxtaposition,  and  firmly  con- 
nected together  by  iron  bolts  and  straps, — or  open,  being  formed 
of  two  beams,  with  an  interval  between  them,  so  connected  by 
cross  and  diagonal  pieces,  that  a  strain  upon  either  the  upper  or 
lower  beam  will  be  transmitted  to  the  other,  and  the  whole  system 
act  under  the  effect  of  the  strain  like  a  solid  beam. 

502.  Solid  built  Beams.  In  framing  solid  built  beams,  the 
pieces  in  each  course  (Fig.  57)  are  laid  abutting  end  to  end  with 

Fig.  57 — Represents  a  solid  built  beam 
of  three  courses,  the  pieces  of 
each  course  breaking  joints  and 
confined  by  iron  hoops. 

a  square  joint  between  them,  the  courses  breaking  joints  to  form 
a  strong  bond  between  them.  The  courses  are  firmly  connected 
either  by  iron  bolts,  formed  with  a  screw  and  nut  at  one  end  to 
bring  the  courses  into  close  contact,  or  else  by  iron  bands  driven 
on  tight,  or  by  iron  stirrups  (Fig.  58)  suitably  arranged  with  screw 
ends  and  nuts  for  the  same  purpose. 


— 1 

1 

— 1 

! 

— i 

—J 

— , 

— 

JL 

— . 

A 


Fig.  58 — Represents  an  iron  stirrup,  or  hoop  a  with  nuts  or  fermv'i 
screws  c  which  confine  the  cross  piece  of  the  stirrup  b 


When  the  strain  is  of  such  a  character  that  the  courses  would 
be  liable  to  work  loose  and  slide  along  their  joints,  the  beams  of 
the  different  courses  may  be  made  with  shallow  indentations, 
(Figs.  59,  60,)  accurately  fitting  into  each  other;  or  shallow  rec- 


Fig.  59 — Represents  a  solid  built 
beam  of  three  courses  arranged 
with  indents  and  confined  by  iron 
hoops. 


Fig.  60  -Represents  a  solid  built  beam,  the  top  part  being  of  two  pieces  b,  b  which  abut 
against  a  broad  flat  iron  bolt  a,  termed  a  king  bolt. 

langular  notches  (Fig.  61)  may  be  cut  across  each  beam,  being 


172 


FRAMING. 


bo  placed  as  to  receive  blocks,  or  keys  of  hard  wood.     The  keyi 


a 


Fig.  61— Represents  a  solid  biUi 
beam  with  keys  b,  b  of  hard  towi 
between  the  courses. 


are  sometimes  made  of  two  wedge-shaped  pieces,  (Fig.  62,)  for 


I 


I 


Fig.  62— Represents  the  keys  in  the  form  of 
double,  or  folding  wedges  a,  b  let  into  a  shal- 
low notch  in  the  beam  c. 


the  purpose  of  causing  them  to  fit  the  notches  more  closely,  and 
to  admit  of  being  driven  tight  upon  any  shrinkage  of  the  woody 
fibre. 

The  joints  between  the  courses  may  be  left  slightly  open 
without  impairing  in  an  appreciable  degree  the  strength  of  the 
combination.  This  is  a  good  method  in  beams  exposed  to  mois- 
ture, as  it  allows  of  evaporation  from  the  free  circulation  of  the 
air  through  the  joints.  Felt,  or  stout  paper  saturated  with  min- 
eral tar,  has  been  recommended  to  secure  the  joints  from  the 
action  of  moisture.  The  prepared  material  is  so  placed  as  to 
occupy  the  entire  surface  of  the  joint,  and  the  whole  is  well 
screwed  together. 

503.  Open  built  Beams.  In  framing  open  built  beams,  the 
principal  point  to  be  kept  in  view  is  to  form  such  a  connection 
between  the  upper  and  lower  solid  beams,  that  they  shall  be 
strained  uniformly  by  the  action  of  a  strain  at  any  point  between 
the  bearings.     This  may  be  effected  in  various  ways,  (Fig.  63.) 

Fig.  83 — Represents  an  open 
built  beam ;  A  and  B  are 
the  top  and  bottom  rails  or 
strings  ;  a,  a,  cross  pieces, 
either  single  or  in  pairs;  b, 
iliaeonal  braces  in  pairs;  c, 
single  diagonal  braces. 

The  upper  and  lower  beams  may  consist  either  of  single  beams, 
or  of  solid  built  beams ;  these  are  connected  at  regular  intervals 
by  pieces  at  right  angles  to  them,  between  which  diagonal  pieces 
are  placed.  By  this  arrangement  the  relative  position  of  all  the 
parts  of  the  frame  will  be  preserved,  and  the  strain  at  any  point 
will  be  brought  to  bear  upon  the  intermediate  points. 

Two  of  the  best  known  applications  of  this  combination,  when 
timber  alone  is  used,  are  those  of  Colonel  Long,  of  the  U.  S. 
Topographical  Engineers,  and  of  the  late  Mr.  Town. 

504.  That  of  Colonel  Long  (Fig.  64)  consists  in  forming  both 
the  upper  and  lower  beams,  termed  by  the  inventor  the  strings, 


FRAMING 


173 


of  three  parallel  beams,  sufficient  space  being  left  between  the 
one  in  the  centre  and  the  other  two  to  insert  the  cross  pieces, 


Fig.  64 — Represents  a  panel  of  Long's  truss. 

A  and  B,  top  and  bottom  strings  of  three  courses. 

C,  C,  posts  in  pairs. 

D,  braces  in  pairs. 

E,  counter  brace  single. 

a,  a,  mortises  where  jibs  and  keys  are  inserted 

F,  jib  and  key  of  hard  wood. 

termed  the  posts ;  the  posts  consist  of  beams  in  pairs  placed  at 
suitable  intervals  along  the  strings,  with  which  they  are  connected 
by  wedge  blocks,  termed  jibs  and  keys,  which  are  inserted  into 
rectangular  holes  made  through  the  strings,  and  fitting  a  corre- 
sponding shallow  notch  cut  into  each  post.  A  diagonal  piece,  termed 
a  brace,  connects  the  top  of  one  post  with  the  foot  of  the  one  ad- 
jacent by  a  suitable  joint.  Another  diagonal  piece,  termed  the 
counter-brace,  is  placed  crosswise  between  the  two  braces  and 
their  posts,  with  its  ends  abutting  against  the  centre  beam  of  the 
upper  and  lower  strings.  The  counter-braces  are  connected 
with  the  posts  and  braces  by  wooden  pins,  termed  tree-nails. 

In  wide  bearings,  the  strings  will  require  to  be  made  of  several 
beams  abutting  end  to  end ;  in  this  case  the  beams  must  break 


174  PEAKING. 

•omts,  and  short  beams  must  be  inserted  between  the  centre  anr; 
exterior  beams  wherever  the  joints  occur,  to  strengthen  them. 

The  beams  in  this  combination  are  all  of  uniform  cross  section, 
the  joints  and  fastenings  are  of  the  simplest  kind,  and  the  parts 
are  well  distributed  to  call  into  play  the  strength  of  the  string 
and  to  produce  uniform  stiffness  and  strain. 

505.  The  combination  of  Mr.  Town  (Fig.  65)  consists  in  two 
b      a 

)a 

_ .  65 — Represents  an  elevation  A,  and  end  view 
B,  of  a  portion  of  Town's  ti-uss. 

a,  a,  top  strings. 

b,  b,  bottom  strings. 

c,  c,  diagonal  braces 

b 


main  strings,  each  formed  of  two  or  three  parallel  beams  of  two 
thicknesses  breaking  joints.  Between  the  parallel  beams  are  in- 
serted a  series  of  diagonal  beams  crossing  each  other.  These 
diagonals  are  connected  with  the  strings  and  with  each  other  by 
tree-nails.  When  the  strings  are  formed  of  three  parallel  beams, 
diagonal  pieces  are  placed  between  the  centre  and  exterior  beams, 
and  two  intermediate  strings  are  placed  between  the  two  courses 
of  diagonals. 

This  combination,  commonly  known  as  the  lattice  truss,  is  of 
very  easy  mechanical  execution,  the  beams  being  of  a  uniform 
cross  section  and  length.  The  strains  upon  it  are  borne  by  the 
tree-nails,  and  when  used  for  structures  subjected  to  variable 
strains  and  jars,  it  loses  its  stiffness  and  sags  between  the  points 
of  support.  It  is  more  recommendable  for  its  simplicity  than 
scientific  combination. 

506.  A  third  method,  called  after  the  patentee,  Hold's  tru-o, 
has  within  a  few  years  come  into  general  notice.  It  consists  of 
(Fig.  66)  an  upper  and  lower  string,  each  formed  of  several  thick- 
nesses of  beams  placed  side  by  side  and  breaking  joints.  On  the 
upper  side  of  the  lower  string  and  the  lower  side  of  the  upper, 
blocks  of  hard  wood  are  inserted  into  shallow  notches  ;  the  blucks 
are  bevelled  off  on  each  side  to  form  a  suitable  point  of  support, 
or  step  for  the  diagonal  pieces.  One  series  of  the  diagonal  pieces 
are  arranged  in  pairs,  the  others  are  single  and  placed  between 
those  in  pairs.  Two  strong  bolts  of  iron,  which  pass  through 
the  blocks,  connect  the  upper  and  lower  strings,  and  are  arranged 
with  a  screw  cut  on  one  end  and  a  nut  to  draw  the  parts  closely 
together. 

This  combination  presents  a  judicious  arrangement  of  the  parts 
The  blocks  give  abutting  surfaces  for  the  braces  superior  to  those 


FRAMING 


175 


obtained  by  the  ordinary  forms  of  joint  for  this  purpose.     The 
bolts  replace  advantageously  the  timber  posts,  and  in  case  of  the 


Fig.  66—  Represent* 
an  elevation  of  a 
portion  of  Howe's 
truss. 

a,  top  string. 

b,  bottom  strings. 

c,  c,  diagonal  traces 
in  pairs. 

d,  single  braces. 

e,  e,  steps  of  hard 
wood  for  braces. 

/, /,  iron  rods  with 
nuts  and  screws 


rrame  working  toose  and  sagging,  their  arrangement  for  tighten- 
ing up  the  parts  is  simple  and  efficacious.  The  timber  of  each 
string  is  not  combined  to  give  as  great  strength  as  its  cross  sec- 
tion is  susceptible  of,  and  the  lower  string,  upon  which  a  strain 
of  tension  is  brought,  against  which  timber  offers  the  greatest 
resistance,  has  received  a  greater  cross  section  than  that  of  the 
upper. 

The  preceding  combinations  have  been  applied  .generally  in 
our  country  to  bridges.  In  this  application,  the  timber  support- 
ing the  roadway  of  the  bridge  is  usually  placed  on  the  iowei 
strings  ;  two,  three,  or  four  built  beams  being  used,  as  the  case 
may  require,  for  supporting  the  transverse  beams  under  the  road- 
way, the  centre  beams  leaving  an  equal  width  of  roadway  between 
them  and  the  exterior  beams. 

507.  Framing  for  intermediate  Supports.  Beams  of  ordinary 
dimensions  may  be  used  for  wide  bearings  when  intermediate 
supports  can  be  procured  between  the  extreme  points. 

The  simplest  and  most  obvious  method  of  effecting  this  is  to 
^.ace  upright  beams,  termed  props,  or  shores,  at  suitable  intervals 
under  the  supported  beam. 

When  the  props  would  interfere  with  some  other  arrangement, 
;ind  points  of  support  can  be  procured  at  the  extremities  below 
those  on  which  the  beam  rests,  inclined  struts  (Fig.  67)  may  be 
u.".ed.     The  struts  must  have  a  suitably  formed  step  at  the  foot, 

M  be  connected  at  top  with  the  beam  by  a  suitable  joint. 

In  some  cases  the  bearing  may  be  diminished  by  placing  ob 


FR  VMING. 


Fig.  67— Represents  a  horizontal  beam  C  snp» 
ported  near  the  middle  by  inclined  strut! 
A,  A 


the  points  of  suppr  rt  si  ort  pieces,  termed  corbels,  (Fig.  68,)  uid 
supporting  these  near  tl»er  ends  by  struts. 


Fig.  68— Represents  a 
horizontal  beam  c  sup- 
ported by  vertical  post 
a,  a,  with  corbel  pie 
ces  d,  d  and  inclined 
struts  e,  e  to  diminish 
the  bearing. 


In  other  cases  a  portion  of  the  beam,  at  the  middle,  may  be 
strengthened  by  placing  under  it  a  short  beam,  called  a  straining 
beam,  (Fig.  69,)  against  the  ends  of  which  the  struts  abut. 


Fig.  69 — Represents  a 
horizontal  beam  c, 
strengthened  by  a 
straining  beam  /  and 
inclined  struts  e,  e. 


Whenever  the  bearing  may  require  it  the  two  preceding  ar- 
rangements (Fig.  70)  may  be  used  in  connection. 


F  g.  70— Re- 
presents   a 

combination 
of  Figs.  68 
and  69 


In  all  combinations  with  struts,  a  lateral  thrust  will  be  thrown 
on  the  point  of  support  where  the  foot  of  the  strut  rests.  This 
strain  must  be  provided  for  in  arranging  the  strength  of  the  sup- 
ports. 

508.  When  intermediate  supports  can  be  procured  only  above 
the  beam,  an  arrangement  must  be  made  which  shall  answer  the 
purpose  of  sustaining  the  beam  at  its  intermediate  points  by  sus- 
pension. The  combination  will  depend  upon  the  number  of  in- 
termediate points  required. 


FRAMING. 


177 


When  the  beam  requires  to  be  supported  only  at  the  middle, 
it  may  be  done  by  placing  two  inclined  pieces,  resting  on  the 
beam' at  its  extremities,  and  meeting  under  an  angle  above  it, 
from  which  the  middle  of  the  beam  can  be  suspended  by  a  rod  of 
iron,  or  by  another  beam.  If  the  suspending  piece  be  of  iron,  it 
must  be  arranged  at  one  end  with  a  screw  and  nut.  When  the 
support  is  of  timber,  a  single  beam,  called  a  king  post,  (Fig.  71,) 


Fig.  71 — Represents  a  hori- 
zontal beam  c  supported 
in  its  middle  by  a  king 
post  g  suspended  front 
the  struts  e,  e. 


may  be  used,  against  the  head  of  which  the  two  inclined  pieces 
may  abut ;  the  foot  of  the  post  is  connected  with  the  beam  by 
a  bolt,  an  iron  stirrup,  or  a  suitable  joint.  Instead  of  the  ordinary 
king  post,  two  beams  may  be  used ;  these  are  placed  opposite  to 
each  other  and  bolted  together,  embracing  between  them  the  sup- 
ported beam  and  the  heads  of  the  inclined  beams  which  fit  into 
shallow  notches  cut  into  the  supporting  beams.  Pieces  arranged 
in  this  manner  for  suspending  portions  of  a  frame  receive  the 
name  of  suspension  pieces,  or  bridle  pieces. 

When  two  intermediate  points  of  support  are  required,  they  may 
be  obtained  by  two  inclined  pieces  resting  on  the  ends  of  the 
beam  and  abutting  against  the  extremities  of  a  short  horizontal 
straining  beam,  (Fig.  72.)     The  suspension  pieces  in  this  case 


Fig.  72 — Represents  a  beam  e 
supported  at  two  points  by 
posts  g,  g  suspended  from  the 
struts  e,  e  and  straining  beam 

At 


may  be  either  posts,  termed  queen  posts,  arranged  like  a  king 
post,  iron  rods,  or  bridle  pieces.  This  combination  may  be  used 
for  very  wide  bearings,  (Fig.  73,)  by  suitably  increasing  the  num- 
ber of  inclined  pieces  and  straining  beams. 

Some  of  the  preceding  combinations  maybe  used  for  support- 
ing one  end  of  abeam  subjected  to  a  cross  strain  when  the  other 
has  a  fixed  point  of  support.  This  may  be  done  either  by  an  in- 
clined strut  beneath,  or  an  inclined  tie  above  the  beam.  When 
a  wooden  tie  is  used  it  should  consist  of  two  pieces  bolted  to- 
gether and  embracing  the  beam. 

23 


178 


Fig.  78 — Represents  a  beam  c  suspended  from  a  combination  of  struts  and  strain- 
ing beams  by  posts  g,  g. 

509.  The  classifications  under  the  two  preceding  heads  repre- 
sent the  principal  combinations  of  straight  beams  applied  to  the 
purposes  of  framing.  The  frame  of  an  ordinary  roof  presents  one 
of  the  simplest  combinations  by  which  the  action  of  the  different 
parts  of  a  frame  may  be  illustrated. 

A  roof  of  the  ordinary  form  consists  of  two  equally  inclined 
sides  of  metal,  slate,  or  other  material,  which  is  attached  to  a 
covering  of  boards  that  rests  upon  the  frame  of  the  roof.  The 
frame  consists  of  several  vertical  frames,  termed  the  trusses  of 
the  roof,  which  are  placed  parallel  to  and  at  suitable  intervals 
from  each  other  ;  these  receive  horizontal  beams  termed  jyurl  i?is, 
which  rest  upon  them  and  are  placed  at  suitable  intervals  apart, 
and  upon  the  purlins  are  placed  inclined  pieces  termed  the  long 
rafters,  to  which  the  boards  are  attached. 

The  truss  of  a  roof,  for  ordinary  bearings,  consists  (Fig.  74) 


Fig.  74 — Represents  a  roof  truss  for  medium  spans. 

a,  tie  beam  of  truss. 

b,  6,  principal  rafters  framed  into  tie  beam  and  the  king  post  c,  and  confined  at  their 

foot  by  an  iron  strap. 
d,  d,  struts. 
«,  e,  purlins  supporting  the  common  rafters// 

of  a  horizontal  beam  termed  the  tie  learn,  with  which  the  inclined 
beams,  termed  thevrmcipal  rafters,  are  connected  by  suitable 
j  ointe.  The  principal  rafters  may  either  abut  against  each  other 
at  the  top  or  ridge,  or  against  a  king  post.  Inclined  struts  are 
in  some  cases  placed  between  the  principal  rafters  and  kingpost, 
with  which  they  are  connected  by  suitable  joints. 
For  wider  bearings  the  short  rafters  (Fig.  75)  abut  against  a 


FRAMING. 


179 


straining  beam  at  top.     Queen  posts  connect  these  pieces  with 
the  tie-beam.     A  king  post  connects  the  straining  beam  witn  the 


Fig.  75 — Represents  a  roof    truss   for  wide 
spans. 

a,  tie  beam. 

b,  6,  principal  rafters. 

c,  short  rafters  abutting  against  the  straining 
beam  d. 

e  and/  king  and  queen  posts  in  pairs. 

g,  g,  purlins  supporting  common  rafters  h. 


top  of  the  short  rafters  ;  and  struts  are  placed  at  suitable  points 
between  the  rafters  and  king  and  queen  posts. 

In  each  of  these  combinations  the  weight  of  the  roof  covering 
and  the  frames  is  supported  by  the  points  of  support.  The  prin- 
cipal rafters  are  subjected  to  cross  and  longitudinal  strains,  arising 
from  the  weight  of  the  roof  covering  and  from  their  reciprocal  ac- 
tion on  each  other.  These  strains  are  transmitted  to  the  tie  beam, 
causing  a  strain  of  tension  upon  it.  The  struts  resist  the  cross 
strain  upon  the  rafters  and  prevent  them  from  sagging ;  and  the 
king  and  queen  posts  prevent  the  tie  and  straining  beams  from  sag- 
ging and  give  points  of  support  to  the  struts.  The  short  rafters 
and  straining  beam  form  points  of  support  which  resist  the  cross 
str  'in  on  the  principal  rafters,  and  support  the  strain  on  the  queen 
posts. 

510.  Wooden  Arches.  A  wooden  arch  may  be  formed  by 
bending  a  single  beam  (Fig.  76)  and  confining  its  extremities  to 


Fig.  76 — Represents  a  horizontal  beam 
o  supported  at  its  middle  point  by  * 
bent  beam  b. 


prevent  it  from  resuming  its  original  shape.  A  beam  in  this 
state  presents  greater  resistance  to  a  cross  strain  than  when 
straight,  and  may  be  used  with  advantage  where  great  stiffness  is 
required,  provided  the  points  of  support  are  of  sufficient  strength 
to  resist  the  lateral  thrust  of  the  beam.  This  method  can  be 
resorted  to  only  in  narrow  bearings. 

For  wide  arches  a  curved  built  beam  must  be  adopted ;  and 
for  this  purpose  a  solid,  (Figs.  77  and  78,)  or  an  open  built  beam 
may  be  used,  depending  on  the  bearing  to  be  spanned  by  the 
arch.  In  either  case  the  curved  beams  are  built  in  the  same 
manner  as  straight  beams,  the  pieces  of  which  they  are  formed 
being  suitably  bent  to  conform  to  the  curvature  of  the  arch,  which 
may  be  done  either  by  steaming  the  pieces,  by  mechanical  power, 
or  by  the  usual  method  of  softening  the  woody  fibres  by  keeping 
the  pieces  wet  while  subjected  to  the  heat  of  a  light  blaze. 


180 


FRAMING. 


Fig.  77 — Represents  a  wooden  arch  A  formed  of  a  solid  built  beam  of  three 
courses  which  support  the  beams  c,  c  by  the  posts  g,  g  which  are  formed  of 
pieces  in  pairs. 

b,  o,  inclined  struts  to  strengthen  the  arch  by  relieving  it  of  a  part  of  the 
load  on  the  beams  c,  c. 


Fig.  78 — Represents  a  wooden  arch  of  a  solid  built  beam  A  which  supports 
the  horizontal  beam  B  by  means  of  the  posts  a.  a.  The  arch  is  let  into 
the  beam  B  which  acts  as  a  tie  to  confine  its  extremities. 

"Wooden  arches  may  also  be  formed  by  fastening  together  sev- 
eral courses  of  boards,  giving  the  frame  a  polygonal  form,  (Fig. 
79,)  corresponding  to  the  desired  curvature,  and  then  shaping  the 

IF 


Mir7?/ 


Fig.  79 — Represents  an  elevation  A  of  a  wooden 
arch  formed  of  short  pieces  a,  b  which  abut 
end  to  end  and  break  joints. 

B  represents  a  perspective  view  of  this  combina- 
tion, showing  the  manner  in  which  the  parts 
are  keyed  together. 


Erah 


outer  and  inner  edges  of  the  arch  to  the  proper  curve, 
course  is  formed  of  boards  cut  into  sharp  lengths,  depending  on 
the  curvature  required  ;  these  pieces  abut  end  to  end,  the  joints 
being  in  the  direction  of  the  radii  of  curvature,  and  the  pieces 
composing  the  diiferent  courses  break  joints  with  each  other. 
The  courses  may  be  connected  either  by  jibs  and  keys  of  hard 
wood,  or  by  iron  bolts.  This  method  is  very  suitable  for  all 
light  frame  work  where  the  presure  borne  is  not  great. 

Wooden  arches  are  chiefly  used  for  bridges  and  roofs.     They 


FRAMING.  181 

serve  as  intermediate  points  of  support  for  the  framing  on  which 
the  roadway  rests  in  the  one  case,  and  the  roof  covering  in  the 
other.  In  bridges  the  roadway  may  lie  either  above  the  arch, 
or  below  it ;  in  either  case  vertical  posts,  iron  rods,  or  bridles 
connect  the  horizontal  beams  with  the  arch. 

oil.  The  greatest  strain  in  wooden  arches  takes  place  between 
the  crown  and  springing  line  ;  this  part  should,  therefore,  when 
practicable,  be  relieved  of  the  pressure  that  it  would  directly 
receive  from  the  beams  above  it  by  inclined  struts,  so  arranged 
as  to  throw  this  pressure  upon  the  lateral  supports  of  the  arch. 

The  pieces  which  compose  a  wooden  arch  may  be  bent  into 
any  curve.  The  one,  however,  usually  adopted  is  an  arc  of  a 
circle,  as  the  most  simple  for  the  mechanical  construction  of  the 
framing,  and  presenting  all  desirable  strength. 

612.  Centres.  The  wooden  frame  with  which  the  voussoirs 
of  an  arch  are  supported  while  the  arch  is  in  progress  of  con- 
struction is  termed  a  centre. 

A  centre,  like  the  frame  of  a  roof,  consists  of  a  number  of 
vertical  frames  (Figs.  80,  81,  82,  83,)  termed  trusses,  or  ribs, 
upon  which  horizontal  beams,  termed  holsters,  are  placed  to  re- 
ceive the  voussoirs  of  the  arch. 

The  curved,  or  back  pieces  of  a  centre  on  which  the  bolsters 
rest  consist  of  beams  cut  into  suitable  lengths  and  shaped  to  the 
proper  curvature ;  these  pieces  abut  end  to  end,  the  joints  be- 
tween them  being  in  the  direction  of  the  radii  of  curvature  ;  the 
joints  are  usually  secured  by  short  pieces,  or  blocks  placed  un- 
cler  the  abutting  ends  to  which  the  back  pieces  are  bolted.  The 
blocks  form  abutting  surfaces  for  shores,  or  inclined  struts  seated 
against  firm  points  of  support  below  the  back  pieces.  To  pre- 
vent the  shores,  or  the  struts  from  bending,  braces,  or  bridles, 
which  are  usually  formed  of  two  pieces,  each  with  shallow  notches 
cut  into  them,  are  added,  and  embrace  between  them  the 
shores,  or  struts,  the  whole  being  firmly  connected  with  iron  bolts. 

The  combinations  used  for  the  frames  of  centres  will  depend  up- 
on the  position  of  the  points  of  support  and  the  size  of  the  arches. 


Fig.  80 — Represents  the  rib  of  a  centre  for 

light  arches. 
«,  a.  rib  formed  as  in  Fig.  T9. 
b,  b,  bolster  pieces  which  receive  the  masonry. 


513.  For  small  light  arches  (Fig.  80)  the  ribs  may  be  formed 


182 


FBAMIXG. 


of  two  or  more  thicknesses  of  short  boards,  firmly  nai  ed  togeth- 
er ;  the  boards  in  each  course  abutting  end  to  end  by  a  joint  in  the 
direction  of  the  radius  of  curvature  of  the  arch,  and  breaking 
joints  with  those  of  the  other  course.  The  ribs  are  shaped  to 
the  form  of  the  intrados  of  the  arch,  to  receive  the  bolsters, 
which  are  of  battens  cut  to  suitable  lengths  and  nailed  to  the  ribs. 
514.  For  heavy  arches  with  wide  spans,  when  firm  interme- 
diate points  of  support  can  be  procured  between  the  abutments, 
the  back  pieces  (Fig.  81)  may  be  supported  by  shores  placed 

Fig.  81 — Represents  the 
rib  of  a  centre  with  in- 
termediate points  of  sup- 
port 

a,  back  pieces  of  the  rib 
which  receive  the  bol- 
sters/ 

b,  b,  struts  which  support 
the  back  pieces. 

e,  e,  braces. 

c,  solid  beam  resting  on 
the  intermediate  sup- 
ports d,  d,  which  re- 
ceive the  ends  of  the 
struts  b,  b. 

under  the  blocks  in  the  direction  of  the  radii  of  curvature  of 
the  arch,  or  of  inclined  struts  (Fig.  82)  resting  on  the  points  of 


Fig.  32 — Represents  a  part  of  the  rib  of  Grosvenor  Bridge  over  the  Dee  at  Chester.    Span  200 

feet 
A,  A,  intermediate  points  of  support 

a,  a,  a,  struts  resting  upon  cast  iron  sockets  on  the  supports  A. 
bt  b,  two  courses  of  plank  each  4>£  inches  thick  bent  over  the  struts  a,  a,  to  the  form  of  the  arch, 

the  courses  breaking  joints. 
c,  c,  folding  wedges  laid  upon  the  back  pieces  b  of  each  rib  to  receive  the  bolsters  on  which  the 

voussoirs  are  laid. 

support.     The  shores,  or  struts,  are  prevented  from  bending  by 
"braces  suitably  placed  for  the  purpose. 


FUAMIXG. 


183 


515.  It'  intermediate  points  of  support  cannot  be  obtained,  a 
broad  framed  support:  must  be  made  at  each  abutment  to  receive 
the  extremities  of  the  struts  that  sustain  the  back  pieces.  The 
framed  support  (Fig.  83)  consists  of  a  heavy  beam  laid  either 


Fig.  88— Represents  a  part  of  a  rib  of  Waterloo  Bridge  over  the  Thames. 

a,  a,  and  h.  three  heavy  beams,  forming  the  striking  plates,  which  with  the  shores  A,  h,  form 

the  framed  support  for  the  struts  of  the  centre. 
c,  e,  struts  abutting  against  the  blocks  g,  g  placed  under  the  joints  of  the  back  pieces// 
rf,  d,  bridle  or  radial  pieces  in  pairs  which  are  confined  at  top  and  bottom  between  the  horizontal 

ties  ft,  n  of  the  ribs,  also  in  pairs. 
e,  e,  cast  iron  sockets. 
w»,  m,  bolsters  of  the  centre  resting  on  the  back  pieces  / 

horizontally,  or  inclined,  and  is  placed  at  that  joint  of  the  arch, 
(the  one  which  makes  an  angle  of  about  30°  with  the  horizon,) 
where  the  voussoirs,  if  unsupported  beneath,  would  slide  on 
their  beds.  This  beam  is  borne  by  shores  which  find  firm  points 
of  support  on  the  foundations  of  the  abutment. 

The  back  pieces  of  the  centre  (Fig.  83)  may  be  supported  by 
inclined  struts  which  rest  immediately  upon  the  framed  support, 
one  of  the  two  struts  under  eacli  block  resting  upon  one  of  the 
framed  supports,  the  other  on  the  one  on  the  opposite  side,  the  two 
struts  being  so  placed  as  to  make  equal  angles  with  the  radius  of 
curvature  of  the  arch  drawn  through  the  middle  point  of  the 
block.  Bridle  pieces,  placed  in  the  direction  of  the  radius  of 
curvature,  embrace  the  blocks  and  struts  in  the  usual  manner, 


184 


FRAMING. 


and  prevent  the  latter  from  sagging.  This  combination  presents 
a  figure  of  invariable  form,  as  the  strain  at  any  one  point  is 
received  by  the  struts  and  transmitted  directly  to  the  fixed  points 
of  support.  It  has  the  disadvantage  of  requiring  beams  of  great 
length  when  the  span  of  the  arch  is  considerable,  and  of  present- 
ing frequent  crossing  of  the  struts  where  notches  will  be  re- 
quisite, and  the  strength  of  the  beams  thereby  diminished. 

The  centre  of  Waterloo  Bridge  over  the  Thames  (Fig.  83) 
was  framed  on  this  principle.  To  avoid  the  inconveniences  re- 
sulting from  the  crossing  of  the  struts,  and  of  building  beams 
of  sufficient  length  where  the  struts  could  not  be  procured  from 
a  single  beam,  the  device  was  imagined  in  this  work  of  receiv- 
ing the  ends  of  several  struts  at  the  points  of  crossing  into  a 
large  cast-iron  socket  suspended  by  a  bridle  piece. 

516.  When  the  preceding  combination  cannot  be  employed,  a 
strong  truss,  (Fig.  84,)  consisting  of  two  inclined  struts  resting 


Fie.  84 — Represents  a  frame 
lor  a  rib  in  which  the  two 
inclined  struts  b,  b  and  the 
straining  beams  o  form  inter- 
mediate supports  for  some  of 
the  struts  that  support  the 
back  pieces  a,  <i. 

6  and  d  are  the  framed  extreme 
supports. 


!§9g*f 


upon  the  framed  supports,  and  abutting  at  top  against  a  strain- 
ing beam,  may  be  formed  to  receive  the  ends  of  some  of  the 
struts  which  support  the  back  pieces.  This  combination,  and 
all  of  a  like  character,  require  that  the  arch  should  not  be 
constructed  more  rapidly  on  one  side  of  the  centre  than  on  the 
other,  as  any  inequality  of  strain  on  the  two  halves  of  the 
centre  would  have  a  tendency  to  change  the  shape  of  the  frame, 
thrusting  it  in  the  direction  of  the  greater  strain. 

517.  Means  used  for  striking  Centres.  When  the  arch  is 
completed  the  centres  are  detached  from  it,  or  struck.  To  eifect 
this  in  large  centres  an  arrangement  of  wedge  blocks  is  used, 
termed  the  striking  plates,  by  means  of  which  the  centre  may  be 
gradually  lowered  and  detacned  from  the  soffit  of  the  arch.  This 
arrangement  consists  (Fig.  83)  in  forming  steps  upon  the  upper 
surface  of  the  beam  which  forms  the  framed  support  to  receive  a 
wedge-shaped  block,  on  which  another  beam,  having  its  under 
surface  also  arranged  with  steps,  rests.  The  struts  of  the  rib 
either  abut  against  the  upper  surface  of  the  top  beam,  or  else  are 
inserted  into  cast-iron  sockets,  termed  shoe-plates,  fastened  to 


FRAMING.  185 

this  surface.    The  centre  is  struck  by  driving  back  tie  wedge 
block. 

518.  When  the  struts  rest  upon  intermediate  supports  be- 
tween the  abutments,  double,  or  folding  wedges  may  be  placed 
under  the  struts,  or  else  upon  the  back  pieces  of  the  ribs  under 
each  bolster.  The  latter  arrangement  presents  the  advantage 
of  allowing  any  part  of  the  centre  to  be  eased  from  the  soffit, 
instead  of  detaching  the  whole  at  once  as  in  the  other  methods 
of  striking  wedges.  This  method  was  employed  for  the  centres 
of  Grosvenor  Bridge,  (Fig.  82,)  over  the  river  Dee  at  Chester, 
and  was  perfectly  successful  both  in  allowing  a  gradual  settling 
of  the  arch  at  various  points,  and  in  the  operation  of  striking. 

519.  Ties  and  Braces  for  detached  Frames.  When  a  series 
of  frames  concur  to  one  end,  as,  for  example,  the  main  beams 
of  a  bridge,  the  trusses  of  a  roof,  ribs  of  a  centre,  &c,  they 
require  to  be  tied  together  and  stiffened  by  other  beams  to 
prevent  any  displacement,  and  warping  of  the  frames.  For  this 
purpose  beams  are  placed  in  a  horizontal  position  and  notched 
upon  each  frame  at  suitable  points  to  connect  the  whole  together; 
while  others  are  placed  crossing  each  other,  in  a  diagonal  direc- 
tion, between  each  pair  of  frames,  with  which  they  are  united 
by  suitable  joints,  to  stiffen  the  frames  and  prevent  them  from 
yielding  to  any  lateral  effort.  Both  the  ties  and  the  diagonal 
braces  may  be  either  of  single  beams,  or  of  beams  in  pairs,  so 
arranged  as  to  embrace  between  them  the  part  of  the  frames 
with  which  they  are  connected. 

520.  Joints.  The  form  and  arrangement  of  joints  will 
depend  upon  the  relative  position  of  the  beams  joined,  and  the 
object  of  the  joint. 

Joints  may  be  required  for  various  purposes,  either  to  connect 
the  ends  of  beams  of  which  the  axes  are  in  the  same  right  line, 
or  make  an  angle  between  them ;  or  the  end  of  one  beam  with 
the  face  of  another ;  or  where  the  face  of  one  beam  rests  upon 
that  of  another. 

In  all  arrangements  of  joints,  the  axes  of  the  beams  connected 
should  lie  in  tine  same  plane  in  which  the  strain  upon  the  frame 
acts  ;  and  the  combination  should  be  so  arranged  that  the  parts 
will  accurately  fit  when  the  frame  is  put  together,  and  that 
any  portion  may  be  displaced  without  disconnecting  the  rest. 
The  simplest  forms  most  suitable  to  the  object  in  view  will 
usually  be  found  to  be  the  best,  as  offering  the  most  facility  in 
obtaining  an  accurate  fit  of  the  parts. 

In  adjusting  the  surfaces  of  the  joints  an  allowance  should  be 
made  for  any  settling  in  the  frame  which  may  arise  either  from 
the  shrinking  of  the  timber  in  seasoning  while  in  the  frame,  or 
from  the  fibres  yielding  to  the  action  of  the  strain.    This  is  don« 

24 


186  FRAMING. 

by  leaving  sufficient  play  in  the  joints  when  the  frame  is  first  set 
up,  to  admit  of  the  parts  coming  into  perfect  contact  when  the 
frame  has  attained  its  final  settling.  Joints  formed  of  plane  sur- 
faces present  more  difficulty  in  this  respect  than  curved  joints, 
as  the  bearing  surfaces  in  the  latter  case  will  remain  in  contact 
should  any  slight  change  take  place  in  the  relative  positions  of 
the  beams  from  settling ;  whereas  in  the  former  a  slight  settling 
might  cause  the  strains  to  be  thrown  upon  a  corner,  or  edge  of 
the  joint,  by  which  the  bearing  surfaces  might  be  crushed,  and 
the  parts  of  the  frame  work  wrenched  asunder  from  the  leverage 
which  such  a  circumstance  might  occasion. 

The  surface  of  a  joint  subjected  to  pressure  should  be  as 
great  as  practicable,  to  secure  the  parts  in  contact  from  being 
crushed  by  the  strain ;  and  the  surface  should  be  perpendicular 
to  the  direction  of  the  strain  to  prevent  sliding. 

A  thin  sheet  of  wrought  iron,  or  lead,  may  be  inserted 
between  the  surfaces  of  joints  where,  from  the  magnitude  of 
the  strain,  one  of  them  is  liable  to  be  crushed  by  the  other,  as  in 
the  case  of  the  end  of  one  beam  resting  upon  the  face  of  another. 

521.  Folding  wedges,  and  pins,  or  tree-nails,  of  hard  wood 
are  used  to  bring  the  surfaces  of  joints  firmly  to  their  bearings, 
and  retain  the  parts  of  the  frame  in  their  places.  The  wedges 
are  inserted  into  square  holes,  and  the  pins  into  anger-holes 
made  through  the  parts  connected.  As  the  object  of  these 
accessories  is  simply  to  bring  the  parts  connected  into  close 
contact,  they  should  be  carefully  driven  in  order  not  to  cause 
a  strain  that  might  crush  the  fibres. 

To  secure  joints  subjected  to  a  heavy  strain,  bolts,  straps,  and 
hoops  of  wrought  iron  are  used.  These  should  be  placed  in  the 
best  direction  to  counteract  the  strain  and  present  the  parts  from 
separating ;  and  wherever  the  bolts  are  requisite  they  should 
be  inserted  at  those  points  which  will  least  weaken  the  joint. 

522.  Joints  of  Beams  united  end  to  end.  When  the  axes  of 
the  beams  are  in  the  same  right  line,  the  form  of  the  joint  will 
depend  upon  the  direction  of  the  strain.  If  the  strain  is  cme  of 
compression,  the  ends  of  the  beams  may  be  united  by  a  square 
joint  perpendicular  to  their  axes,  the  joint  being  secured  (Fig.SS) 


. , * . 


1 


a     \  \b 


beams  a  and  b  is  fished  oi 


Fig.  85. — Represents  the  manner  in    which  the  end  joint  of  two  bi 
secured  by  side  pieces  c  and  a  bolted  to  them. 

by  four  short  pieces  so  placed  as  to  embrace  the  ends  of  the 
beams,  and  being  fastened  to  the  beams  and  to  each  other  by 


FRAMING. 


187 


bolts.  This  arrangement,  termed  fishing  a  beam,  is  used  only 
for  rough  work.  It  may  also  be  used  when  the  strain  is  one  of 
extension ;  in  this  case  the  short  pieces  (Fig.  86)  may  be  notched 


h—wm- 


-ftiB- 


d  ii 


5 


Fig.  86— Represents  a  fished  joint  in  which  the  side  pieces  c  and  d  are  either  let  into  the 
beams  or  secured  by  keys  e,  e. 

upon  the  beams,  or  else  keys  of  hard  wood,  inserted  into  shallow 
notches  made  in  the  beams  and  short  pieces,  may  be  employed 
to  give  additional  security  to  the  joint. 

A  joint  termed  a  scarf  may  be  used  for  either  of  the  foregoing 
purposes.  This  joint  may  be  formed  either  by  halving  the  beams 
on  each  other  near  their  ends,  (Fig.  87,)  and  securing  the  joints 


£ 


-*=5^ 


L-]- 


d    ; 


Fig.  87 — Represents  a  scarf  joint  secured  by  iron  plates  c,  c,  keys  d,  d,  and  bolts. 

by  bolts,  or  straps ;  or  else  by  so  arranging  the  ends  of  the  two 
beams  that  each  shall  fit  into  shallow  triangular  notches  cut 
into  the  other,  the  joint  being  secured  by  iron  hoops.  This  last 
method  is  employed  for  round  timber. 

523.  When  beams  united  at  their  ends  are  subjected  to  a  cross 
strain,  a  scarf  joint  is  generally  used,  the  under  part  of  the  joint 
being  secured  by  an  iron  plate  confined  to  the  beams  by  bolts. 
The  scarf  for  this  purpose  may  be  formed  simply  by  halvingthe 
beams  near  their  ends ;  but  a  more  usual  and  better  form  (Fig. 


E3 


— m* 


c 


Fig.  8S — Represents  a  scarf  joint  for  a  cross  strain  secured  at  bottom  by  a  piece  of  timber  e 
confined  to  the  beams  by  iron  hoops  d,  d  and  keys  6,  e. 

88)  is  to  make  the  portion  of  the  joint  at  the  top  surface  of 
the  beams  perpendicular  to  their  axes,  and  about  one  third  of 
their  depth ;  the  bottom  portion  being  oblique  to  the  axis,  ae 
well  as  the  portion  joining  these  two. 

"When  the  beams  are  subjected  to  a  cross  strain  and  to  one  of 
extension  in  the  direction  of  their  axes,  the  form  of  the  scarf 
must  be  suitably  arranged  to  resist  each  of  these  strains.  The  one 
shown  in  Fig.  89  is  a  suitable  and  usual  form  for  these  objects.  A 
folding  wedge  key  of  hard  wood  is  inserted  into  a  space  left 


188 


FRAMING. 


between  the  parts  of  the  joint  which  catch  when  the  beams  aie 
drawn  apart.  The  key  serves  to  bring  the  surfaces  of  the  joints 


> 


_iLsr~^ 


<l. 


6 1 


Jk- 


Fie.  89 — Represents  a  scarf  joint  arranged  to  resist  a  cross  strain  and  one  of  extension. 
The  bottom  of  the  joint  is  6ecured~by  an  iron  plate  confined  by  bolts.  The  folding 
wedge  key  inserted  at  c  serves  to  bring  all  the  surfaces  of  the  joints  to  their  bearings. 

to  their  bearings,  and  to  form  an  abutting  surface  to  resist  the 
strain  of  extension.  In  this  form  of  scarf  the  surface  of  the 
joint  which  abuts  against  the  key  will  be  compressed  ;  the 
portions  of  the  beams  just  above  and  below  the  key  will  be 
subjected  to  extension.  These  parts  should  present  the  same 
amount  of  resistance,  or  have  an  equality  of  cross  section.  The 
length  of  the  scarf  should  be  regulated  by  the  resistance  with 
which  the  timber  employed  resists  detrusion  compared  with 
its  resistance  to  compression  and  extension. 

524.  When  the  axes  of  beams  form  an  angle  between  them, 
they  may  be  connected  at  their  ends  either  by  halving  them 
on  each  other,  or  by  cutting  a  mortise  in  the  centre  of  one 
beam  at  the  end,  and  shaping  the  end  of  the  other  to  fit  into  it. 

525.  Joints  for  connecting  the  end  of  one  beam,  with  the  face 
of  another.  The  joints  used  for  this  purpose  are  termed  mortise 
and  tenon  joints.  Their  form  will  depend  upon  the  angle  be- 
tween the  axes  of  the  beams.  When  the  axes  are  perpendicular 
the  mortise  (Fig.  90)  is  cut  into  the  face  of  the  beam,  and  the  end 


Lir 

\_a_U 


i  i 


^ 


U=fc 


Fig.  SO — Represents  a  mortise  and  tenon  joint 
when  the  axes  of  the  beams  are  perpendi- 
cular to  each  other. 

a,  tenon  on  the  beam  A. 

'/.  mortise  in  the  beam  B. 

•.  pin  to  hold  the  parts  together. 


of  the  other  beam  is  shaped  into  a  tenon  to  fit  the  mortise.  When 
the  axes  of  the  beams  are  oblique  to  each  other,  a  triangular 
notch  (Fig.  91)  is  usually  cut  into  the  face  of  one  beam,  the  sides 
of  the  notch  being  perpendicular  to  each  other,  and  a  shallow 
mortise  is  cut  into  the  lower  surface  of  the  notch  ;  the  end  of 
the  other  beam  is  suitably  shaped  to  fit  the  notch  and  mortise. 


FRAMING. 


189 


Tenon  and  mortise  joints  have  received  a  variety  of  forms, 
The  direction  of  the  strain  and  the  effect  it  may  produce  upon 


Fig.  91 — Represents  a  mortise  and  tenon  joint 
when  the  axes  of  the  beams  are  oblique  to 
each  other,  A  notch  whose  surfaces  ab  and 
be  are  at  right  angles  is  cut  into  the  beam  B 
and  a  shallow  mortise  d  is  cut  below  the  sur- 
face be.  The  end  of  the  beam  A  is  arranged 
to  fit  the  notch  and  mortise  in  B.  The  joint 
is  secured  by  a  screw  bolt. 


the  j  oint  must  in  all  cases  regulate  this  point.  In  some  cases  the 
circular  joint  may  be  more  suitable  than  those  forms  which  are 
plane  surfaces  ;  in  others  a  double  tenon  may  be  better  than  the 
simple  joint. 

526.  Tie  joints.  These  joints  are  used  to  connect  beams 
which  cross,  or  lie  on  each  other.  The  simplest  and  strongest 
form  of  tie  joint  consists  in  cutting  a  notch  in  one,  or  both  of  the 
beams  to  connect  them  securely.  But  when  the  beams  do  not 
cross,  but  the  end  of  one  rests  upon  the  other,  a  notch  of  a  tra- 
pezoidal form  (Fig.  92)  may  be  cut  in  the  lower  beam  to  receive 


Ays 


& 


TFig.  92- 
\      pin  at 


92— Represents  an  ordinary  dove-tail  joint  secured  by  a 


the  end  of  the  upper,  which  is  suitably  shaped  to  fit  the  notch. 
This,  from  its  shape,  is  termed  a  dove-tail  joint.  It  is  of  fre- 
quent use  in  joinery,  but  is  not  suitable  for  heavy  frames  where 
the  joints  are  subjected  to  considerable  strains,  as  it  soon  becomes 
loose  from  the  shrinking  of  the  timber. 

527.  Iron  Frames.  Cast  and  wrought  iron  are  both  used  for 
frames.  The  former  is  most  suitable  where  great  strength  com- 
bined with  stiffness  is  required  ;  the  latter  for  light  frames  and 
wherever  the  strains  act  mainly  as  tensions. 

In  iron  frames  the  same  general  principles  of  combination 
are  applicable  as  in  those  of  timber,  and  they  admit  of  the  same 
classification  as  frames  of  the  latter  material. 

Cast  iron  is  most  easily  wrought  into  the  best  forms  for 
strength.  The  dimensions  of  the  pieces  must,  however,  be  re- 
stricted within  certain  practical  limits,  both  on  account  of  the 


190 


FRAMING. 


labor  and  expense  attendant  npon  the  casting  and  handling  of 
heavy  pieces,  and  the  difficulty  of  procuring  them  of  uniform 
quality  when  of  large  size.  In  arranging  the  component  parts  of 
an  iron  frame,  uniformity  in  the  6hape  and  dimensions  is  requi- 
site both  for  economy  and  perfection  of  workmanship  ;  and  as  far 
as  practicable,  the  bulk  of  the  different  parts  of  each  piece  should 
be  the  same,  in  order  to  avoid  the  dangers  arising  from  unequal 
shrinking  in  cooling. 

Wrought  iron  may  be  hammered,  or  rolled  into  the  most  suit- 
able form  for  strength,  but  for  frames  bars  of  a  rectangular  sec- 
tion are  mostly  used. 

The  join {s  in  both  cast  and  wrought  iron  frames  are  made  upon 
the  same  principles  as  in  those  of  timber,  the  forms  being  adapted 
to  the  nature  of  the  material ;  they  are  secured  by  wrought  iron 
wedges,  keys,  bolts,  &c. 

528.  Vra/mesfor  Cross  Strains.  Solid  beams  of  cast  iron, 
moulded  into  the  most  suitable  forms  for  strength  and  for  adap- 
tation to  the  object  in  view,  may  be  used  for  supporting  a  cross 
strain  where  the  bearings  are  of  a  medium  width.  Solid  wrought 
iron  beams  can  be  used  with  economy  for  the  same  purposes 
only  for  short  bearings. 

529.  Open  cast  iron  beams  are  seldom  used  except  in  combina- 
tion with  cast  iron  arches.  Those  of  wrought  iron  are  frequently 
used  in  structures.  They  may  be  formed  of  a  top  and  bottom 
rail  connected  by  diagonal  pieces,  forming  the  ordinary  lattice 
arrangement ;  or  a  piece  bent  into  a  curved  form  may  be  placed 


Fig.  93 — Represents  an  open  beam  of 
wrought  Iron  consisting  of  a  top  and 
bottom  rail  a  and  6,  with  an  inter- 
mediate curved  piece,  the  whole 
secnred  by  the  pieces  c,  c  in  pairs 
bolted  to  thee 

d,  e,  and ,/  represent  the  parts  of  a 
truss  of  a  curved  light  roof,  con- 
nected with  the  open  beam;  and 
also  the  manner  in  which  the  whole 
are  secured  to  the  walL 


between  the  rails,  or  any  other  suitable  combination  (Fig.  93) 
may  be  used  which  combines  lightness  with  strength  and  stiffness. 


FRAMING. 


191 


530.  Iron  Arches.  Cast  iron  arches  may  be  used  for  the  same 
objects  as  those  of  timber.  The  frames  for  these  purposes  con- 
sist of  several  parallel  ribs  of  uniform  dimensions  which  are  cast 
into  an  arch  form,  the  ribs  being  connected  by  horizontal  ties, 
and  stiffened  by  diagonal  braces.  The  weight-  of  the  superstruc- 
ture is  transmitted  to  the  curved  ribs  in  a  variety  of  ways ;  most 
usually  by  an  open  cast  iron  beam,  the  lower  part  of  which  is  so 
shaped  as  to  rest  upon  the  curved  rib,  and  the  upper  part  suitably 
formed  for  the  object  in  view.  These  beams  are  also  connected 
by  tics,  and  stiffened  by  diagonal  braces. 

Each  rib,  except  for  narrow  spans,  is  composed  of  several 
pieces,  or  segments,  between  each  pair  of  which  there  is  a  joint 
in  the  direction  of  the  radius  of  curvature.  The  forms  and  di- 
mensions of  the  segments  are  uniform.  The  segments  are  usually 
either  solid,  (Fig.  94,)  or  open  plates  of  uniform  thickness,  having 


Fig.  94  —  Repre- 
sents a  portion 
of  a  cast  iron 
plate  arch  with 
an  open  cast  iron 
beam. 

A,  A,  segments  of 
the  arch. 

B,  B,  panels  of  the 
open  beam  con- 
nected at  the 
joints  ab. 


a  flanch  of  uniform  breadth  and  depth  at  each  end,  and  on  the 
entrados  and  intrados.  The  flanch  serves  both  to  give  strength 
to  the  segment  and  to  form  the  connection  between  the  segments 
and  the  parts  which  rest  upon  the  rib. 

The  ribs  are  connected  by  tie  plates  which  are  inserted  be- 
tween the  joints  of  the  segments,  andare  fastened  to  the  segments 
by  iron  screw  bolts  which  pass  through  the  end  flanches  of  the 
segments  and  the  tie  plate  between  them.  The  tie  plates  may  be 
either  open,  or  solid ;  the  former  being  usually  preferred  on  ac- 
count of  this  superior  lightness  and  cheapness. 

The  frame  work  of  the  ribs  is  stiffened  by  diagonal  pieces 
which  are  connected  either  with  the  ribs,  or  the  tie  plates.  The 
diagonal  braces  are  cast  in  one  piece,  the  arms  being  ribbed,  or 


192 


FRAMING. 


feathered^  and  tapering  from  the  centre  towards  the  ends  in  a 
suitable  manner  to  give  lightness  combined  with  strength. 

The  open  beams  (Fig.  94)  which  rest  upon  the  curved  ribs  are 
cast  in  a  suitable  number  of  panels  ;  the  joint  between  each  pair 
being  either  in  the  direction  of  the  radii  of  the  arch,  or  else  verti- 
cal. These  pieces  are  also  cast  with  Handles,  by  which  they  are 
connected  together  and  with  the  other  parts  of  the  frame.  The 
beams,  like  the  ribs,  are  tied  together  and  stiffened  by  ties  and 
diagonal  braces. 

Beams  of  suitable  forms  for  the  purposes  of  the  structure  are 
placed  either  lengthwise,  or  crosswise  upon  the  open  beams. 

531.  Curved  ribs  of  a  tubular  form  have,  within  a  few  years 
back,  been  tried  with  success,  and  bid  fair  to  supersede  the  or- 
dinary plate  rib,  as  with  the  same  amount  of  metal  they  combine 
more  strength  than  the  flat  rib. 

The  application  of  tubular  ribs  was  first  made  in  the  United 


Fig.  95—  Represents  a  side  view  A,  and  a  cross  section  and  end  view  B  through  a  saddle  piece  of 

the  tubular  arch  of  Major  Delafield. 
a,  a,  (Fig  A)  a  side  view,  and  (Fig.  B)  an  end  view  of  the  elliptical  flanches  of  the  end  of  each 

segment. 
h,  b,  shoulders,  or  ribs  to  strengthen  the  flanches  against  lateral  strains. 

c,  tie  plate  between  the  ribs. 

/  (Fig^B)  side  view  of  the  rim  cf  the  tie-plate  fitted  to  the  interior  of  the  tube. 

d,  d,  (Figs.  A  and  B)  saddle  pieces  to  receive  the  open  beams  of  a  form  similar  to  Fig  94,  which 

rest  on  the  tubular  ribs. 
«,  cross  section  of  the  rib  through  the  saddle  piece. 


FRAMING. 


193 


States  by  Major  Delafield  of  the  U.  S.  Corps  of  Engineers,  in 
an  arch  for  a  bridge  of  80  feet  span.  Each  rib  was  formed  of 
nine  segments  ;  each  segment  (Fig.  95)  being  cast  in  one  piece, 
the  cross  section  of  which  is  an  elliptical  ring  of  uniform  thick- 
ness, the  transverse  axis  of  the  ellipse  being  in  the  direction  of 
the  radios  of  curvature  of  the  rib.  A  broad  elliptical  flanch 
witli  ribs,  or  stays,  is  cast  on  each  end  of  the  segment,  to  connect 
the  parts  with  each  other ;  and  three  chairs,  or  saddle  pieces, 
with  grooves  in  them,  are  cast  upon  the  entrados  of  each  seg- 
ment, and  at  equal  intervals  apart,  to  receive  the  open  beam 
which  rests  on  the  curved  rib. 

The  ribs  are  connected  by  an  open  tie  plate,  (Fig.  95.)  Raised 
elliptical  projections  are  cast  on  each  face  of  the  tie  plate,  where 
it  is  connected  with  the  segments,  which  are  adjusted  accurately 
to  the  interior  surface  of  each  pair  of  segments,  between  which 
the  tie  plate  is  embraced.  The  segments  and  plate  are  fastened 
by  screw  bolts  passed  through  the  end  flanches  of  the  segments. 

The  tie  plates  form  the  only  connection  between  the  curved 
ribs ;  the  broad  ribbed  flanches  of  the  segments,  and  the  raised 
rims  of  the  tie  plates  inserted  into  the  ends  of  the  tubes,  giving 
all  the  advantages  and  stiffness  of  diagonal  pieces. 

532.  Tubular  ribs  with  an  elliptical  cross  section  have  been 
used  in  France  for  many  of  their  bridges.  They  were  first  intro- 
duced but  a«few  years  back  by  M.  Polonceau,  after  whose 


<£> 


® 


<°> 


<D 


p-    G 


[>_ 


Fig.  96— Represents  a  side  view  A  and  a  cross  section  and  end  view  B  through  a  joint  of  M. 

Polonceau's  tubular  arch. 
a,  a,  top  flanch,  b,  b  bottom  flanch  of  the  semi-segments  united  along  the  vertical  joint  cd  through 

the  axis  of  the  rib. 
gh,  side  view  of  the  joint  between  the  flanches  «,  e  of  two  semi-segments. 
m,  inner  side  of  the  flanches. 

c,  cross  section  of  a  semi-segment  and  top  and  bottom  flanches. 
//  thin  weiljres  of  wrought  iron  placed  oetween  the  end  flanches  of  the  semi-segments  to  bring 

the  parts  to  their  proper  bearing. 

25 


194 


FRAMING. 


designs  the  greater  part  of  these  structures  have  been  "built. 
According  to  M.  Polonceau's  plan,  each  rib  consists  of  two 
symmetrical  parts  divided  lengthwise  by  a  vertical  joint  Each 
half  of  the  rib  is  composed  of  a  number  of  segments  so  distribut- 
ed as  to  break  joints,  in  order  that  when  the  segments  are  put 
together  there  snail  be  no  continuous  cross  joint  through  the  ribs. 

The  segments  (Fig.  96)  are  cast  with  a  top  and  bottom  flanch 
and  one  also  at  each  end.  The  halves  of  the  rib  are  connected 
by  bolts  through  the  upper  and  lower  flanches,  and  the  segments 
by  bolts  through  the  end  flanches. 

For  the  purposes  of  adjusting  the  segments  and  bringing  the 
rib  to  a  suitable  degree  of  tension,  flat  pieces  of  wrought  iron  of 
a  wedge  shape  are  driven  into  the  joints  between  the  segments, 
and  are  confined  in  the  joints  by  the  bolts  which  fasten  the 
segments  and  which  also  pass  through  these  wedges. 

To  connect  the  ribs  with  each  other,  iron  tubular  pieces  are 


Fie.  9T — Represents  the  half  of  a  truss  of  wrought  iron  for  the  new  Houses  of  Parliament,  England. 
The  pieces  of  this  truss  are  formed  of  bars  of  a  rectangular  section.  The  joints  are  secnr«d  by 
cast  iron  sockets,  within  which  the  ends  of  the  bars  are  secured  by  screw  bolts. 


FRAMING. 


195 


placed  between  them,  the  ends  of  the  tubes  being  suitably  ad- 
justed to  the  sides  of  the  ribs.  Wrought  iron  rods  which  serve 
as  ties  pass  through  the  tubes  and  ribs,  being  arranged  with 
screws  and  nuts  to  draw  the  ribs  firmly  against  the  tubular  pieces. 
Diagonal  pieces  of  a  suitable  form  are  placed  between  the  ribs 
to  give  them  the  requisite  degree  of  stiffness. 

In  the  bridges  constructed  by  Mr.  Polonceau  according  to 
this  plan,  he  supports  the  longitudinal  beams  of  the  roadway  by 
cast  iron  rings  which  are  fastened  to  the  ribs  and  to  each  other, 
and  bear  a  chair  of  suitable  form  to  receive  the  beams. 

533.  Iron  roof  T?iisses.  Frames  of  iron  for  roofs  have  been 
made  either  entirely  of  wrought  iron,  or  of  a  combination  of 
wrought  and  cast  iron,  or  of  these  two  last  materials  combined 
with  timber.  The  combinations  for  the  trusses  of  roofs  of  iron 
are  in  all  respects  the  same  as  in  those  for  timber  trusses.  The 
parts  of  the  truss  subjected  to  a  cross  strain,  or  to  one  of  corn- 


Fig.  98 — Represents  the  half  of  a  trass  for  the  same  building  composed  of  wroifght  and  cast 
iron. 

a,  a,  feathered  struts  of  cast  iron. 

b,  b,  suspension  bars  in  pairs. 
*»,  *t,  tie  and  straining  bars. 

e,  «,  and//;  oross  sections  of  beams  resting  in  the  cast  iron  sockets  connected  with  the  suspen- 
sion bars. 


196 


FKAM1JNG. 


pression,  are  arranged  to  give  the  most  suitable  forms  for 
strength,  and  to  adapt  them  to  the  object  in  view.  The  parte 
subjected  to  a  strain  of  extension,  as  the  tie-beam  and  king  and 
queen  posts,  are  made  either  of  wrought  iron  or  of  timber,  as  may 
be  found  best  adapted  to  the  particular  end  proposed.  The  joints 
are  in  some  cases  arranged  by  inserting  the  ends  of  the  beams, 
or  bars,  in  cast  iron  sockets,  or  shoes  of  a  suitable  form ;  in 
others  the  beams  are  united  by  joints  arranged  like  those  for 
timber  frames,  the  joints  in  all  cases  being  secured  by  wrought 
iron  bolts  and  keys.     (Figs.  97,  98,  and  99.) 


Fig.  99 — Represents  the  arrange- 
ments of  the  parts  at  the  joint 
c  in  Fig.  98. 

A,  side  view  of  the  pieces  and 
joint 

a,  principal  rafter  of  the  cross 
section  B. 

b,  common  rafter  of  the  cross 
section  C. 

c,  cross  section  of  purlins  and 
joint  for  fastening  the  com- 
mon rafters  to  the  purlins. 

d,  cast  iron  socket  arranged 
to  confine  the  pieces  a,  b, 
e,  «. 


534.  Flexible  Supports  for  Frames.  Chains  and  ropes  may 
frequently  be  substituted  with  advantage,  for  rigid  materials,  as 
intermediate  points  of  support  for  frames,  forming  systems  of 
suspension  in  which  the  parts  supported  are  suspended  from 
the  flexible  supports,  or  else  rest  upon  them  either  directly,  or 
through  the  intermedium  of  rigid  beams. 

535.  All  systems  of  suspension  are  based  upon  the  property 
which  the  catenary  curve  in  a  state  of  equilibrium  possesses 
of  converting  vertical  pressures  upon  it  into  tensions  in  the  di- 
rection of  the  curve.  These  systems  therefore  offer  the  advan- 
tages of  presenting  the  materials  of  which  they  are  composed 
in  the  best  manner  for  calling  into  action  the  greatest  amount 
of  resistance  of  which  they  are  capable,  and  of  allowing  the 
dimensions  of  the  parts  to  be  adapted  to  the  strain  thrown  upon 
them  more  accurately  than  can  be  done  in  rigid  systems ;  thus 
avoiding  much  of  the  unproductive  weight  necessarily  intro- 
duced into  structures  of  stone,  wood,  and  cast  iron.  They  offer 
also  the  farther  advantages  that  in  their  construction  the  parts 
of  which  they  are  composed  can  be  readily  adjusted,  put  together, 


FRAMING. 


and  taken  apart  for  repairs.  They  present  the  disadvai.  tages  of 
changing  both  their  form  and  dimensions  from  the  action  of  the 
weather  and  variations  of  temperature,  and  of  being  liable  to 
grave  accidents  from  undulations  and  vertical  vibrations  caused 
by  high  winds,  or  moveable  loads.  The  require,  therefore,  that 
the  fixed  points  of  support  of  the  system  should  be  very  firm 
and  durable,  and  that  constant  attention  should  be  given  to 
keep  the  system  in  a  thorough  state  of  repair. 

536.  A  chain  or  rope,  when  fastened  at  each  extremity  to 
fixed  points  of  support,  will,  from  the  action  of  gravity,  assume 
the  form  of  a  catenary  in  a  state  of  equilibrium,  whether  the 
two  extremities  be  on  the  same,  or  different  levels.  The  rela- 
tive height  of  the  fixed  supports  may  therefore  be  made  to 
conform  to  the  locality. 

537.  The  ratio  of  the  versed  sine  of  the  arc  to  its  chord,  or 
span,  will  also  depend,  for  the  most  part,  on  local  circumstances 
and  the  object  of  the  suspended  structure.  The  wider  the  span, 
or  chord,  for  the  same  versed  sine,  the  greater  will  be  the 
tension  along  the  curve,  and  the  more  strength  will  therefore 
be  required  in  all  the  parts.  The  reverse  will  obtain  for  an 
increase  of  versed  sine  for  the  same  span ;  but  there  will  be  an 
increase  in  the  length  of  the  curve. 

538.  The  chains  may  either  be  attached  at  the  extremities  of 
the  curve  to  the  fixed  supports,  or  piers ;  or  they  may  rest  upon 
them,  (Fig.  100,  101,)  being  fixed  into  anchoring  masses,  or 


& 

iff/ 


;i__ ■.,,-.! ^r.^lilllMX1 

m 

'.  iWimiW PWWBWB^S  ^^wsiSSWii 

Fig.  100 — Represents  a  chain  arch  abode,  resting  upon  two  piers  f,  f  and  anchored  at  the  points 
a  and  e,  from  which  a  horizontal  beam  mm.  is  suspended  by  vertical  chains,  or  rods. 


Fig.  101 — Represents  the  manner  in  which  the  system  may  be  arranged  when  a  single  pier  is 
placed  between  the  extreme  points  of  the  bearing. 

abutments,  at  some  distance  beyond  the  piers.  Local  circum- 
stances will  determine  which  of  the  two  methods  will  be  the 
more  suitable.  The  latter  is  generally  adopted,  particularly  if 
the  piers  require  to  be  high,  since  the  strain  upon  them  from 
the  tension  might,  from  the  leverage,  cause  rupture  in  the  pier 
near  the  bottom,  and  because,  moreover,  it  remedies  in  some 


198  FRAMING. 

degree  the  inconveniences  arising  from  variations  of  tensior- 
caused  either  by  a  moveable  load,  or 'changes  of  temperature. 
Piers  of  wood,  or  of  cast  iron  moveable  around  a  joint  at  their 
base,  have  been  used  instead  of  fixed  piers,  with  the  object 
of  remedying  the  same  inconveniences. 

539.  'Vv'hen  the  chains  pass  over  the  piers  and  are  anchored 
at  some  distance  beyond  them,  they  may  either  rest  upon 
saddle  pieces  of  cast  iron,  or  upon  pulleys  placed  on  the  piers. 

540.  The  position  of  the  anchoring  points  will  depend  upon 
local  circumstances.  The  two  branches  of  the  chain  may  either 
make  equal  angles  with  the  axis  of  the  pier,  thus  assuming  the 
same  curvature  on  each  side  of  it,  or  else  the  extremity  of  the 
chain  may  be  anchored  at  a  point  nearer  to  the  base  of  the  pier. 
In  the  former  case  the  resultant  of  the  tensions  and  weights  will 
be  vertical  and  in  the  direction  of  the  axis  of  the  pier,  in  the 
latter  it  will  be  oblique  to  the  axis,  and  should  pass  so  far 
within  the  base  that  the  material  will  be  secure  from  crushing. 

541.  The  anchoring  points  are  usually  masses  of  masonry  of 
a  suitable  form  to  resist  the  strain  to  wnich  they  are  subjected. 
They  may  be  placed  either  above  or  below  the  surface  of  the 
ground,  as  the  locality  may  demand.  The  kind  of  resistance 
offered  by  them  to  the  tension  on  the  chain  will  depend  upon 
the  position  of  the  chain.  If  the  two  branches  of  the  chain  make 
equal  angles  with  the  axis  of  the  pier,  the  resistance  offered 
by  the  abutments  will  mainly  depend  upon  the  strength  of  the 
material  of  which  they  are  formed.  If  the  branches  of  the 
chain  make  unequal  angles  with  the  axis  of  the  pier,  the  branch 
fixed  to  the  anchoring  mass  is  usually  deflected  in  a  vertical 
direction,  and  so  secured  that  the  weight  of  the  abutment  may 
act  in  resisting  the  tension  on  the  chain.  In  this  plan  fixed 
pulleys  placed  on  very  firm  supports  will  be  required  at  the  point 
of  deflection  of  the  chain  to  resist  the  pressure  arising  from 
the  tension  at  these  points. 

Whenever  it  is  practicable  the  abutment  and  pier  should  be  suit- 
ably connected  to  increase  the  resistance  offered  by  the  former. 

The  connection  between  the  chains  and  abutments  should  be 
so  arranged  that  the  parts  can  be  readily  examined.  The  chains 
at  these  points  are  sometimes  imbedded  in  a  paste  of  fat  lime  to 
preserve  them  from  oxidation. 

542.  The  chains  may  be  placed  either  above  or  below  the 
structure  to  be  supported.  The  former  gives  a  system  of  more 
stability  than  the  latter,  owing  to  the  position  of  the  centre  of 
gravity,  but  it  usually  requires  high  piers,  and  the  chain  cannot 
generally  be  so  well  arranged  as  in  the  latter  to  subserve  the  re- 
quired purposes.  The  curves  may  consist  of  one  or  more  chains. 
Several  are  usually  preferred  to  a  single  one,  as  for  the  same 


FRAMING.  199 

amount  of  metal  they  offer  more  resistance,  can  be  more  accu- 
rately manufactured,  are  less  liable  to  accidents,  and  can  be 
more  easily  put  up  and  replaced  than  a  single  chain.  The 
chains  of  the  curve  may  be  placed  either  side  by  side,  or  above 
each  other,  according  to  circumstances. 

543.  The  curves  may  be  formed  either  of  chains,  of  wire  ca- 
bles, or  of  bands  of  hoop  iron.  Each  of  these  methods  has 
found  its  respective  advocates  among  engineers.  Those  who 
prefer  wire  cables  to  chains  urge  that  the  latter  are  more  liable 
to  accidents  than  the  former,  that  their  strength  is  less  uniform 
and  less  in  proportion  to  their  weight  than  that  of  wire  cables, 
that  iron  bare  are  more  liable  to  contain  concealed  defects  than 
wire,  that  the  proofs  to  which  chains  are  subjected  may  increase 
without,  in  all  cases,  exposing  these  defects,  and  that  the  con- 
struction and  putting  up  of  chains  is  more  expensive  and  diffi- 
cult than  for  wire  cables.  The  opponents  of  wire  cables  state 
that  they  are  open  to  the  same  objections  as  those  urged  against 
chains,  that  they  offer  a  greater  amount  of  surface  to  oxidation 
than  the  same  volume  of  bar  iron  would,  and  that  no  precau- 
tion can  prevent  the  moisture  from  penetrating  into  a  wire 
cable  and  causing  rapid  oxidation. 

That  in  this,  as  in  all  like  discussions,  an  exaggerated  degree 
of  importance  should  have  been  attached  to  the  objections  urged 
on  each  side  was  but  natural.  Experience,  however,  derived 
from  existing  works,  has  shown  that  each  method  may  be  ap- 
plied with  safety  to  structures  of  the  boldest  character,  and  that 
wherever  failures  have  been  met  with  in  either  method,  they 
were  attributable  to  those  faults  of  workmanship,  or  to  defects 
in  the  material  used,  which  can  hardly  be  anticipated  and 
avoided  in  any  novel  application  of  a  like  character.  Time 
alone  can  definitively  decide  upon  the  comparative  merits  of  the 
two  methods,  and  how  far  either  of  them  may  be  used  with 
advantage  in  the  place  of  structures  of  more  rigid  materials. 

544.  The  chains  of  the  curves  may  be  formed  of  either  round, 
square,  or  flat  bars.  Chains  of  flat  bars  have  been  most  gene- 
rally used.  These  are  formed  in  long  links  which  are  connected 
by  short  plates  and  bolts.  Each  link  consists  of  several  bars  of 
the  same  length,  each  of  which  is  perforated  with  a  hole  at 
each  end  to  receive  the  connecting  bolts.  The  bars  of  each 
link  are  placed  side  by  side,  and  the  links  are  connected  by 
the  plates  which  form  a  short  link,  and  the  bolts. 

The  links  of  the  portions  of  the  chain  which  rest  upon  the 
piers  may  either  be  bent,  or  else  be  made  shorter  than  the 
others  to  accommodate  the  chain  to  the  curved  form  of  the  sur- 
face on  which  it  rests. 

545.  The  vertical  suspension  bars  may  be  either  of  round  oi 


200  FRAMING. 

square  bars.  They  are  usually  made  with  one  or  more  articu- 
lations, to  admit  of  their  yielding  with  less  strain  to  the  bar  to 
any  motion  of  vibration,  or  of  oscillation.  They  may  be  sus- 
pended from  the  connecting  bolts  of  the  links,  but  the  prefera- 
ble method  is  to  attach  them  to  a  suitable  saddle  piece  which 
is  fitted  to  the  top  of  the  chain  and  thus  distributes  the  strain 
upon  the  bar  more  uniformly  over  the  bolts  and  links.  The 
lower  end  of  the  bar  is  suitably  arranged  to  connect  it  with 
the  part  suspended  from  it. 

546.  The  wire  cables  used  for  curves  are  composed  of  wires 
laid  side  by  side,  which  are  brought  to  a  cylindrical  shape  and 
confined  by  a  spiral  wrapping  of  wire.  To  form  the  cable  seve- 
ral equal  sized  ropes,  or  yarns,  are  first  made.  This  may  be 
done  by  cutting  all  the  wires  of  the  length  required  for  the  yarn, 
or  by  uniting  end  to  end  the  requisite  number  of  wires  for  the 
yarn,  and  then  winding  them  around  two  pieces  of  wrought  or 
of  cast  iron,  of  a  horse-shoe  shape,  with  a  suitable  gorge  to  re- 
ceive the  wires,  which  are  placed  as  far  asunder  as  the  required 
length  of  the  yarn.  The  yarn  is  firmly  attached  at  its  two  ends 
to  the  iron  pieces,  or  cruppers,  and  the  wires  are  temporarily  con- 
fined at  intermediate  points  by  a  spiral  lashing  of  wire.  Whichever 
of  the  two  methods  be  adopted,  great  care  must  be  taken  to  give 
to  every  wire  of  the  yarn  the  same  degree  of  tension  by  a  suitable 
mechanism.  The  cable  is  completed  after  the  yarns  are  placed 
upon  the  piers  and  secured  to  the  anchoring  ropes  or  chains ;  for 
this  purpose  the  temporary  lashings  of  the  yarns  are  undone,  and 
all  the  yarns  are  united  and  brought  to  a  cylindrical  shape  and 
secured  throughout  the  extent  of  the  cable,  to  within  a  short 
distance  of  each  pier,  by  a  continuous  spiral  lashing  of  wire. 

The  part  of  the  cable  which  rests  upon  the  pier  is  not  bound 
with  wire,  but  is  spread  over  the  saddle  piece  with  a  uniform 
thickness. 

547.  The  suspension  ropes  are  formed  in  the  same  way  as  the 
cables ;  they  are  usually  arranged  with  a  loop  at  each  end,  form- 
ed around  an  iron  crupper,  to  connect  them  with  the  cables, 
to  which  they  are  attached,  and  to  the  parts  of  the  structure 
suspended  from  them  by  suitable  saddle  pieces. 

548.  To  secure  the  cables  from  oxidation  the  iron  wires  are 
coated  with  varnish  before  they  are  made  into  yarns,  and  after 
the  cables  are  completed  they  are  either  coated  with  the  usual 
paints  for  securing  iron  from  the  effects  of  moisture,  or  else 
covered  with  some  impermeable  material. 

549.  Experiments  on  the  Strength  of  Frames.  Experimental 
researches  on  this  point  have  been  mostly  restricted  to  those 
made  with  models  on  a  comparatively  small  scale,  owing  to  the 
expense  and  difficulty  attendant  upon  experiments  on  frames 


FRAMING. 


201 


having  the  form  and  dimensions  of  those  employed  in  ordinary 
structures. 

Among  the  most  remarkable  experiments  on  a  large  scale 
are  those  made  by  order  of  the  French  government  at  Lorient, 
under  the  direction  of  M.  Kiebell,  the  superintending  engineer 
of  the  port,  and  published  in  the  Annates  Maritimes  et  Colo 
males,  Feb.  and  Not.,  1837. 

The  experiments  were  made  by  first  setting  up  the  frame  to 
be  tried,  and,  after  it  had  settled  under  the  action  of  its  own 
weight,  suspending  from  the  back  of  it,  by  ropes  placed  at 
equal  intervals  apart,  equal  weights  to  represent  a  load  uni- 
formly distributed  over  the  back  of  the  frame. 

The  results  contained  in  the  following  table  are  from  experi- 
ments on  a  truss  (Fig.  102)  for  the  roof  of  a  ship  shed.  The 
truss  consisted  of  two  rafters  and  a  tie  beam,  with  suspension 

Fig.  102. 


pieces  in  pairs,  and  diagonal  iron  bolts  which  were  added  be- 
cause it  was  necessary  to  scarf  the  tie  beam.  The  span  of  the 
truss  was  65-|  feet ;  the  rafters  had  a  slope  of  1  perpendicular  to 
4  base.  The  thickness  of  the  beams,  measured  horizontally,  was 
about  2-J-  inches,  their  depth  about  18  inches.  The  amount  of 
the  settling  at  each  rope  was  ascertained  by  fixed  graduated 
vertical  rods,  the  measures  being  taken  below  a  horizontal  line 
marked  0. 


WKIOHTS   BOBNE   BT  TUB   TRUSS. 

Amount  of  settling  on  the  right  of 
the  ridge  below  the  horizontal  0, 
in  inches. 

a 

o 

« 

I! 

a  £ 

OO  "^ 

< 

a 
1 

.  6 

c  bo 

"2 

>i   ° 
«*" 

33 

a 

S 
o 

«i| 

00 

-3 
= 

• 

a 

o    . 

*"•  bo 
."O 

G-G 

•a 

i-i 

< 

Weight  uniformly  distributed,  1654  lbs. 

Do.                     do.            8680  lbs. 

Do.                     do.            1654  lbs.  and  1868  lbs.,  sus- 
pended from  the  centre  of  the  frame 
8680  lbs.,  uniformly  distributed,  and  1368  lbs.  from  the 

0.15 
1.6 

0.4 

0.15 
1.7 

0.5 

0.15 
1.9 

0.4 

2.3 

0.15 
1.8 

0.8 

2.1 

0.15 
1.1 

0.2 

1.2 

The  following  table  gives  the  results  of  experiments  made  on 
frames  of  the  usual  forms  of  straight  and  curved  timber  for  roof 
trusses.     The  cur\  ed  pieces  were  made  of  two  thicknesses,  each 

26 


202 


FRAMING. 


3^  inches.  The  numbers  in  the  fifth  column  give  the  ratios 
between  the  weight  of  the  frame  and  that  of  the  weight  borne 
by  which  the  elasticity  was  not  impaired. 


Fig.  108. 


Fig.  104. 


Fig.  108. 


/     /^"      '     ■ 

i 

■r* 

^Ti 

r                         :          ;          :, 

IZSI 

Fig.  106. 


^ 

£&<* 

"""Ir 

3 

FRAMING. 


203 


o 

Mip 

2  **> 

53 

60 

1 

a 

"3 

4 

3 

•a 

■ 

*iiM 

„3 

DESCRIPTION   Or  THE   FRAMES. 

3 

■ 

u 
o 

i 

•°  I 

_  ■» 

3.8 

etween 
of  fra 
the  loo 
borne 
ng  elas 

fa 

o 

a 

■^  M-*->       T  o 

"S  • 

OS 

a. 

<o 

3 
P 

o 

N 

O        a)     be"  © 

-u.2  bo 

"3  1 

s 

o 
H 

5  cs 

Frame  formed  of  two  rafters  and  a  tie  beam  . 

25  ft 

8  ft. 

8.5  in. 

8.1  in. 

14.80 

2600 

8916 

Do.                  do.                      do. 

and  suspension  pieces  in  pairs,  (Fig.  103).    . 
Frame  of  a  segment  arch  confined  Dy  a  tie 

— 

— 

— 

— 

8.83 

2T70 

5520 

54  ft. 

lift. 

12  in. 

Tin. 

8.S5 

6520 

12240 

Do.                   do.                        do. 

with  suspension  pieces  in  pairs,  (Fig.  105)  . 

— 

— 

— 

— 

2.82 

9500 

180T7 

Frame  of  a  segment  arch  with  rafters   con- 

fined at  their  foot  by  a  tie  piece,  (Fig.  106)  . 
Frame  of  a  full  centre  arch  confined  by  a  . 

— 

— 

— 

— 

8.91 

6111 

21896 

50  ft. 

25  ft. 

— 

— 

1.00 

4386 

5161 

Do.                  do.                     do. 

with  suspension  pieces  in  pairs 

~~ 

— 

™ ■ 

■■ 

0.91 

7328 

8158 

Am  Not*  A.,  Appemm. 


204  BRIDGES,  ETC. 


BKIDGES,     &c. 


550.  Under  this  head  will  be  comprised  that  class  of  struc- 
tures whose  object  is  to  afford  a  Hue  of  communication  above 
the  general  surface  of  a  country,  either  by  means  of  a  roadway, 
or  of  a  water-way,  without  obstructing  those  communications 
which  lie  upon  the  surface. 

When  the  structure  supports  a  roadway  it  is  termed  a  viaduct ; 
and  when  a  water-way  an  aqueduct. 

If  the  structure  is  limited  to  affording  a  communication  over 
a  water-course,  it  is  termed  a  bridge  when  it  supports  a  road- 
way, and  an  aqueduct-bridge  when  it  affords  a  water-way. 

For  the  convenience  of  description,  bridges,  &c,  may  be  clas- 
sified either  from  the  kind  of  material  of  which  they  are  con- 
structed, as  a  Stone-Bridge ',  a  Wooden-Bridge,  &c,  or  from  the 
character  of  the  structure,  as  a  Permanent- Bridge,  a  Draw 
Bridge,  &c. 

STONE   BRIDGES. 

551.  A  stone  bridge  consists  of  a  roadway  which  rests  upon 
one  or  more  arches,  usually  of  a  cylindrical  form,  the  abutments 
and  piers  of  the  arches  being  of  sufficient  height  and  strength 
to  secure  them  and  the  roadway  from  the  effects  of  an  extraor- 
dinary rise  in  the  water-course. 

552.  Locality.  The  point  where  a  bridge  may  be  required, 
as  well  as  the  direction  of  the  axis,  or  centre  line  of  the  roadway 
over  the  bridge,  usually  depends  upon  the  position  of  a  line  of 
communication  which  traverses  the  water-course,  and  of  which 
the  bridge  is  a  necessary  link.  When,  however,  the  engineer  is 
not  restricted  in  the  choice  of  a  suitable  locality  by  this  condi- 
tion, he  should  endeavor  to  select  one  where  the  soil  of  the  bed 
will  afford  a  firm  support  for  the  foundations  of  the  structure  : 
where  the  approaches,  or  avenues  leading  from  the  banks  of  the 
watercourse  to  the  bridge  can  be  easily  made,  not  requiring 
high  embankments  or  deep  excavations  ;  and  one  where  the  re- 
gimen of  the  water-course  is  uniform  and  not  likely  to  be 
changed  in  any  hurtful  degree  by  elbows,  or  other  variations 
in  the  water-way  near  the  bridge,  or  by  the  obstruction  which 
the  foundations,  &c,  of  the  structure  may  offer  to  the  free  dis- 
charge of  the  water. 

To  avoid  the  difficulties  which  the  construction  of  askew  arches 


STONE   BRIDGES.  205 

presents,  the  axis  of  the  bridge  should  be  perpendicular  to  the 
direction  of  the  thread  of  the  current,  since  for  the  security  of  the 
foundations,  the  faces  of  the  piers  and  abutments  of  the  arches 
must  be  placed  parallel  to  the  thread  of  the  current. 

553.  Surrey.  With  whatever  considerations  the  locality  may 
have  been  selected,  a  careful  survey  must  be  made  not  only  of 
it,  but  also  of  the  water-course  and  its  environs  for  some  distance 
above  and  below  the  point  which  the  bridge  will  occupy,  to  en- 
able the  engineer  to  judge  of  the  probable  effects  which  the 
bridge  when  erected  may  have  upon,  the  natural  regimen  of  the 
water-course. 

The  object  of  the  survey  will  be  to  ascertain  thoroughly  the 
natural  features  of  the  surface,  the  nature  of  the  subsoil  of  the 
bed  and  banks  of  the  water- course,  and  the  character  of  the 
water-course  at  its  different  phases  of  high  and  low  water,  and 
of  freshets.  This  information  will  be  embodied  in  a  topographi- 
cal map  ;  in  cross  and  longitudinal  sections  of  the  water-course 
and  the  substrata  of  its  bed  and  banks,  as  ascertained  by  sound- 
ings and  borings ;  and  in  a  descriptive  memoir  which,  besides  the 
usual  state  of  the  water-course,  should  exhibit  an  account  of 
its  changes,  occasioned  either  by  permanent  or  by  accidental 
causes,  as  from  the  effects  of  extraordinary  freshets,  or  from 
the  construction  of  bridges,  dams,  and  other  artificial  changes 
either  in  the  bed  or  banks. 

554.  Having  obtained  a  thorough  knowledge  both  of  the  posi- 
tion to  be  occupied  by  the  bridge  and  its  environs,  the  two  most 
essential  points  which  will  next  demand  the  consideration  of  the 
engineer  will  be,  in  the  first  place,  so  to  adapt  his  proposed  struc- 
ture to  thelocality,  that  a  sufficient  water-way  shall  be  leftbothfor 
navigable  purposes  and  for  the  free  discharge  of  the  water  accu- 
mulated during  high  freshets  ;  and,  in  the  second,  to  adopt  such 
a  system  of  foundations  as  will  be  most  likely  to  ensure  the 
safety  of  the  structure  when  exposed  to  this  cause  of  danger. 

555.  Waterway.  When  the  natural  water-way  of  a  river  is 
obstructed  by  any  artificial  means,  the  contraction,  if  consider- 
able, will  cause  the  water,  above  the  point  where  the  obstruction 
is  placed,  to  rise  higher  than  the  level  of  that  below  it,  and  pro- 
duce a  fall,  with  an  increased  velocity  due  to  it,  in  the  current 
between  the  two  levels.  These  causes  during  heavy  freshets, 
may  be  productive  of  serious  injury  to  agriculture,  from  the  over- 
flowing of  the  banks  of  the  water  course  ; — may  endanger,  if  not 
entirely  suspend  navigation,  during  the  seasons  of  freshets; — and 
expose  any  structure  which,  like  a  bridge,  forms  the  obstruction, 
to  ruin,  from  the  increased  action  of  the  current  upon  the  soil 
around  its  foundations.  If,  on  the  contrary,  the  natural  water- 
way is  enlarged  at  the  point  where  the  structure  is  placed,  with 


206  BRIDGES,   ETC. 

the  view  of  preventing  these  consequences,  the  velocity  of  the 
current,  during  the  ordinary  stages  of  the  water,  will  be  de- 
creased, and  tnis  will  occasion  deposits  to  be  formed  at  the 
point,  which,  by  gradually  filling  up  the  bed,  might,  on  a  sudden 
rise  of  the  water,  prove  a  more  serious  obstruction  than  the  struc- 
ture itself;  particularly  if  the  main  body  of  the  water  should  hap- 
pen to  be  diverted  by  the  deposit  from  its  ordinary  channels,  and 
form  new  ones  of  greater  depth  around  the  foundations  of  the 
structure. 

The  water-way  left  by  the  structure  should,  for  the  reasons 
above,  be  so  regulated  that  no  considerable  change  shall  be  oc- 
casioned in  the  velocity  of  the  current  through  it  during  the 
most  unfavorable  stages  of  the  water. 

556.  For  the  purpose  of  deciding  upon  the  most  suitable  ve- 
locity for  the  current  through  the  contracted  water-way  formed 
by  the  structure,  the  velocity  of  the  current  and  its  effects  upon 
the  soil  of  the  banks  and  bed  of  the  natural  water-way  should  be 
carefully  noted  at  those  seasons  when  the  water  is  highest ;  se- 
lecting, in  preference,  for  these  observations,  those  points  above 
and  below  the  one  which  the  bridge  is  to  occupy,  where  the 
natural  water-way  is  most  contracted. 

557.  The  velocity  of  the  current  at  any  point  may  be  ascer- 
tained by  the  simple  process  of  allowing  a  light  ball,  or  float  of 
some  material,  like  white  wax,  or  camphor,  whose  specific  grav- 
ity is  somewhat  less  than  that  of  water,  to  be  carried  along  by 
the  current  of  the  middle  thread  of  the  water-course,  and  noting 
the  time  of  its  passage  between  two  fixed  stations. 

558.  From  the  velocity  at  the  surface,  ascertained  in  this 
way,  the  average,  or  mean  velocity  of  the  water,  which  flows 
through  the  cross-section  of  any  water-way  between  the  stations 
where  the  observations  are  taken,  may  be  found,  by  taking  four 
fifths  of  the  velocity  at  the  surface. 

Having  the  mean  velocity  of  the  natural  water-way,  that  of 
the  artificial  water-way  will  be  obtained  from  the  following  ex- 
pression, 

s 
v  =  m  —  v, 

s 

in  which  s  and  v  represent,  respectively,  the  area  and  mean 
velocity  of  the  artificial  water-way ;  s  and  v,  the  same  data  of 
the  natural  water-way  ;  and  m  a  constant  quantity,  which,  as 
determined  from  various  experiments,  may  be  represented  by  the 
mixed  number  1,097. 

With  regard  to  the  effect  of  the  increased  velocity  on  the  bed, 
there  are  no  experiments  which  directly  apply  to  the  cases  usually 
met  with.    The  following  table  is  drawn  up  from  experiments 


STONE   BRIDGES. 


207 


made  in  a  confined  channel,  the  bottom  and  sides  of  the  channel 
being  formed  of  rough  boards. 


Stages  of  accumu- 
lation termed 

Velocity     of 
river  in  feet 
per  second. 

Nature  of  the  bottom  which  just  bears 
such  velocities. 

Specific  gravi- 
ty of  the  ma- 
terial. 

Ordinary  floods . 

Uniform  tenors . 

Gliding      .    .    . 
Dull      .... 

(3.2 
12.17 
11.07 
•{0.62 
(0.71 
0.351 
0.26 

Angular  stones,  the  size  of  a  hen's  egg    . 
Bounded  pebbles  one  inch  in  diameter  . 
Gravel  of  the  size  of  garden  beans  .    .    . 

Sand,  the  grains  the  size  of  aniseeds    .    . 

2.25 

2.614 

2.545 

2.545 

2.86 

2.545 

2.64 

559.  Bays.  With  the  data  now  before  him,  the  engineer  can 
proceed  to  the  arrangement  of  the  forms  and  details  of  the  va- 
rious parts  of  the  proposed  structure. 

The  first  point  to  be  considered  under  this  head  will  be  the 
number  of  bays,  or  intervals  into  which  the  natural  water-way 
must  be  divided,  and  the  forms  and  dimensions  of  the  arches 
which  span  the  bays. 

As  a  general  rule,  there  should  be  an  odd  number  of  bays, 
whenever  the  width  of  the  water-way  is  too  great  to  be  spanned 
by  a  §ingle  arch.  Local  circumstances  may  require  a  departure 
from  this  canon ;  but  when  departed  from,  it  will  be  at  the  cost 
of  architectural  effect ;  since  no  secondary  feature  can  occupy 
the  central  point  in  any  architectural  composition  without  impair- 
ing the  beauty  of  the  structure  to  the  eye ;  and  as  the  arches 
are  the  main  features  of  a  stone  bridge,  the  central  point  ought 
to  be  occupied  by  one  of  them. 

The  width  of  the  bays  will  depend  mainly  upon  the  charac- 
ter of  the  current,  the  nature  of  the  soil  upon  which  the  founda- 
tions rest,  and  the  kind  of  material  that  can  be  obtained  for  the 
masonry. 

For  streams  with  a  gentle  current,  which  are  not  subject  to 
heavy  freshets,  narrow  bays,  or  those  of  a  medium  size  may  be 
adopted,  because,  even  a  considerable  diminution  of  the  natural 
water-way  will  not  greatly  affect  the  velocity  under  the  bridge, 
and  the  foundations  therefore  will  not  be  liable  to  be  undermined. 
The  difficulty,  moreover,  of  laying  the  foundations  in  streams  of 
this  character  is  generally  inconsiderable.  For  streams  with  a 
rapid  current,  and  which  are  moreover  subject  to  great  freshets, 
wide  bays  will  be  most  suitable,  in  order,  by  procuring  a  wide 
water-way,  to  diminish  the  danger  to  the  points  of  support,  in 
placing  as  few  in  the  stream  as  practicable. 

If  materials  of  the  best  quality  can  be  procured  for  the  struc- 
ture, wide  bays  with  bold  arches  can  be  adopted  with  safety  ; 
but,  if  the  materials  are  of  an  inferior  quality,  it  will  be  most 


208  BRIDGES,    ETC. 

prudent  to  adopt  bays  of  a  small,  or  medium  space,  and  a 
strong  form  of  arch. 

560.  Arches.  Cylindrical  arches  with  any  of  the  usual  forms 
of  curve  of  intrados  may  be  used  for  bridges.  The  selection 
will  be  restricted  by  the  width  of  the  bay,  the  highest  water- 
level  during  freshets,  the  approaches  to  the  bridge,  and  the 
architectural  effect  which  may  be  produced  by  the  structure,  as 
it  is  more  or  less  exposed  to  view  at  the  intermediate  stages  be- 
tween high  and  low  water. 

Oval  and  segment  arches  are  mostly  preferred  to  the  full  cen- 
tre arch,  particularly  for  medium  and  wide  bays,  for  the  reasons 
that,  for  the  same  level  of  roadway,  they  afford  a  more  ample 
water-way  under  them,  and  their  heads  and  spandrels  offer  a 
smaller  surface  to  the  pressure  of  the  water  during  freshets  than 
the  full  centre  arch  under  like  circumstances. 

The  full  centre  arch,  from  the  intrinsic  beauty  of  its  form,  the 
simplicity  of  its  construction,  and  its  strength,  should  be  preferred 
to  any  other  arch  for  bridges  over  water-courses  of  a  uniformly 
moderate  current,  and  which  are  not  subjected  to  considerable 
changes  in  their  water-levels,  particularly  when  its  adoption  does 
not  demand  expensive  embankments  for  the  approaches. 

If  the  bays  spanned  by  the  arches  are  of  the  same  width,  the 
curves  of  all  the  arches  must  be  identical.  If  the  bays  are  of 
unequal  width,  the  widest  should  occupy  the  centre  of  the  struc- 
ture, and  those  on  each  side  of  the  centre  should  either  be  of 
equal  width,  or  else  decrease  uniformly  from  the  centre  to  each 
extremity  of  the  bridge.  In  this  case  the  curves  of  the  arches 
should  be  similar,  and  have  their  springing  lines  on  the  same 
level  throughout  the  bridge. 

The  level  of  the  springing  lines  will  depend  upon  the  rise  of 
the  arches,  and  the  heiglit  of  their  crowns  above  the  water-level 
of  the  highest  freshets.  The  crown  of  the  arches  should  not,  as 
a  general  rule,  be  less  than  three  feet  above  the  highest  known 
water-level,  in  order  that  a  passage-way  maybe  left  for  floating 
bodies  descending  during  freshets.  Between  this,  the  lowest 
position  of  the  crown,  and  any  other,  the  rise  should  be  so  chosen 
that  the  approaches,  on  the  one  hand,  may  not  be  unnecessarily 
raised,  nor,  on  the  other,  the  springing  lines  be  placed  so  low 
as  to  mar  the  architectural  effect  of  the  structure  during  the 
ordinary  stages  of  the  water. 

When  the  arches  are  of  the  same  size,  the  axis  of  the  roadway 
and  the  principal  architectural  lines  which  run  lengthwise  along 
the  heads  of  the  bridge,  as  the  top  of  the  parapet,  the  cornice, 
&c,  &c,  will  be  horizontal,  and  the  bridge,  to  use  a  common 
expression,  be  on  a  dead  level  throughout.  This  has  for  some 
time  been  a  favorite  feature  in  bridge  architecture,  few  of  the 


STONE  BRIDGES.  209 

more  recent  and  celebrated  bridges  being  without  it,  as  it  is 
thought  to  give  a  character  of  lightness  and  boldness  to  the  struc- 
ture which  is  wanting  in  bridges  built  with  a  uniform  declivity 
from  the  centre  to  the  extreme  arches.  Without  stopping  to 
examine  this  claim  of  architectural  beauty  for  level  bridges,  it 
is  well  to  state  that  it  may  be  purchased  at  too  great  a  cost,  par- 
ticularly in  localities  where  the  relative  level  of  the  roadway 
and  of  the  adjacent  ground  would  demand  high  embankments 
for  the  approaches. 

561.  Style  of  Architecture.  The  design  and  construction  of 
a  bridge  should  be  governed  by  the  same  general  principles  as 
any  other  architectural  composition.  As  the  object  of  a  bridge 
is  to  bear  heavy  loads,  and  to  withstand  the  effects  of  one  of 
the  most  destructive  agents  with  which  the  engineer  has  to 
contend,  the  general  character  of  its  architecture  should  be  that 
of  strength.  It  should  not  only  be  secure,  but  to  the  apprehen- 
sion appear  so.  It  should  be  equally  removed  from  Egyptian  mas- 
siveness  and  Corinthian  lightness ;  while,  at  the  same  time,  it 
should  conform  to  the  features  of  the  surrounding  locality,  being 
more  ornate  and  carefully  wrought  in  its  minor  details  in  a  city, 
and  near  buildings  of  a  sumptuous  style,  than  in  more  obscure 
quarters  ;  and  assuming  every  shade  of  conformity,  from  that 
which  would  be  in  keeping  with  the  humblest  hamlet  and  tamest 
landscape  to  the  boldest  features  presented  by  Xature  and  Art. 
Simplicity  and  strength  are  its  natural  characteristics ;  all  orna- 
ment of  detail  being  rejected  which  is  not  of  obvious  utility,  and 
suitable  to  the  point  of  view  from  which  it  must  be  seen ;  as  well 
as  all  attempts  at  boldness  of  general  design  which  might  give 
rise  to  a  feeling  of  insecurity,  however  unfounded  in  reality.  The 
most,  therefore,  that  can  be  tried  in  the  way  of  mere  ornament, 
even  under  the  most  favorable  circumstances,  will  be  to  combine 
the  voussoirs  of  the  arches  with  the  horizontal  courses  of  the  span- 
drels in  a  regular  and  suitable  manner, — to  add  a  projecting  cor- 
nice, with  supporting  members  if  necessary,  of  an  agreeable  pro- 
file,— and  to  give  such  a  form  to  the  ends  of  the  piers,  termed  the 
starlings,  or  cut-waters,  as  shall  heighten  the  general  pleasing 
effect.  The  heads  of  the  bridge,  the  cornice,  and  the  parapet 
should  also  generally  present  an  unbroken  outline ;  this,  however, 
may  be  departed  from  in  bridges  where  it  is  desirable  to  place  re- 
cesses for  seats,  so  as  not  to  interfere  with  the  footpaths;  in  which 
case  a  plain  buttress  may  be  built  above  each  starling  to  support 
the  recess  and  its  seats,  the  utility  of  which  will  be  obvious,  while 
it  will  give  an  appearance  of  additional  strength  when  the  height 
of  the  parapet  above  the  starlings  is  at  all  considerable. 

562.  Construction.  The  methods  of  laying  the  foundations 
of  structures  of  stone,  &c,  described  under  the  article  of  Ma- 

27 


210  BRIDGES,  ETC. 

Bomy,  being  alike  applicable  to  all  structures  which  come  under 
this  denomination,  there  only  remains  to  be  added  under  this 
head  whatever  is  peculiar  to  bridge-building.  Either  of  the 
methods  referred  to  may  be  employed  in  laying  the  foundations 
of  the  abutments  and  piers  of  a  bridge,  which,  in  the  judgment 
of  the  engineer,  may  be  most  suited  to  the  locality,  and  will  be 
least  expensive.  As  the  foundations  and  their  beds  of  the  parts 
in  question  are  greatly  exposed,  from  the  action  of  the  current 
both  upon  the  soil  around  them  and  upon  the  materials  used 
for  their  construction,  the  utmost  precaution  should  be  taken  to 
secure  them  from  damage,  by  giving  to  the  foundation-bed  an 
:ample  spread  where  the  soil  is  at  all  yielding ;  by  selecting  the 
most  durable  materials  for  the  masonry  of  these  parts  ;  and  by 
employing  some  suitable  means  for  securing  the  bed  of  the 
natural  water-way  around  and  between  the  piers  from  being 
removed  by  the  current. 

563.  Various  expedients  have  been  tried  to  effect  this  last 
object;  among  the  most  simple  and  efficacious  of  which  is  that 
of  covering  the  surface  to  be  protected  by  a  bed  of  stone  broken 
into  fragments  of  sufficient  bulk  to  resist  the  velocity  of  the 
current  in  the  bays,  if  the  soil  is  of  an  ordinary  clayey  mud ; 
but,  if  it  be  of  loose  sand  or  gravel,  the  surface  should  be  first 
covered  by  a  bed  of  tenacious  clay  before  the  stone  be  thrown 
in.  The  voids  between  the  blocks  of  stone,  in  time,  become 
tilled  with  a  deposite  of  mud,  which,  acting  as  a  cement,  gives 
to  the  mass  a  character  of  great  durability. 

564.  The  foundation  courses  of  the  piers  should  be  formed  of 
heavy  blocks  of  cut  stone  bonded  in  the  most  careful  manner, 
and  carried  up  in  offsets.  The  faces  of  the  piers  should  be  of 
cut  stone  well  bonded.  They  may  be  built  either  vertically,  or 
with  a  slight  batter.  Their  thickness  at  the  impost  should  be 
greater  than  what  would  be  deemed  sufficient  under  ordinary 
circumstances ;  as  they  are  exposed  to  the  destructive  action  of 
the  current,  and  of  shocks  from  heavy  floating  bodies ;  and  from 
the  loss  of  weight  of  the  parts  immersed,  owing  to  the  buoyant 
effort  of  the  water,  their  resistance  is  decreased.  The  most  suc- 
cessful bridge  architects  have  adopted  the  practice  of  making 
the  thickness  of  the  piers  at  the  impost  between  one  sixth  and 
one  eighth  of  the  span  of  the  arch.  The  thickness  of  the  piers 
of  the  bridge  of  ISeuilly,  near  Paris,  built  hj  the  celebrated 
Perronet,  whose  works  form  an  epoch  in  moaern  bridge  archi- 
tecture, is  only  one  ninth  of  the  span,  its  arches  also  being  re- 
markable for  the  boldness  of  their  curve. 

565.  The  usual  practice  is  to  give  to  all  the  piers  tr  e  same 
proportional  thickness.  It  has  however  been  recommer  ded  by 
some  engineers  to  give  sufficient  thickness  to  a  few  of  the  piers 


I 

8T0NE  BRIDGES. 


211 


to  resist  the  horizontal  thrust  of  the  arches  on  either  side  of 
them,  and  thus  secure  a  part  of  the  structure  from  ruin,  should 
an  accident  happen  to  any  of  the  other  piers.  These  masses, 
to  which  the  name  abutment  piers  has  been  applied,  would  be 
objectionable  from  the  diminution  of  the  natural  water-way  that 
would  be  caused  by  their  bulk,  and  from  the  additional  cost  for 
their  construction,  besides  impairing  tne  architectural  effect  of 
the  structure.  They  present  the  advantage,  in  addition  to  their 
main  object,  of  permitting  the  bridge  to  be  constructed  by 
sections,  and  thus  procure  an  economy  in  the  cost  of  the  wooden 
centres  for  the  arches. 

566.  The  projection  of  the  starlings  beyond  the  heads  of  the 
bridge,  their  form,  and  the  height  given  to  them  above  the  spring- 
ing lines,  will  depend  upon  local  circumstances.  As  the  main 
objects  of  the  starlings  are  to  form  &  fender ,  or  guard  to  secure 
the  masonry  of  the  spandrels,  &c,  from  being  damaged  by  float- 
ing bodies,  and  to  serve  as  a  cut-water  to  turn  the  current  aside, 
and  prevent  the  formation  of  whirls,  and  their  action  on  the  bed 
around  the  foundations,  the  form  given  to  them  should  subserve 
both  these  purposes.  Of  the  different  forms  of  horizontal  section 
which  have  been  given  to  starlings,  (Figs.  107,  108,  109,  110,) 


Fig.  10T. 


Fig.  108. 


Figs.  107,  108,  and  110— Repre- 
sent horizontal  sections  of 
starlings  A  of  the  more  usual 
forms,  and  part  of  the  pier  B 
above  the  foundation  courses. 
Fig.  109  represents  the  plan  of 
the  hood  of  a  starling  laid  in 
courses,  the  general  shape  be- 
ing that  of  the  quarter  of  a 
sphere. 


the  semi-ellipse,  from  experiments  carefully  made,  with  these 
ends  in  view,  appears  best  to  satisfy  both  objects. 

The  up  and  down  stream  starlings,  in  tidal  rivers  not  subject 
to  freshete  and  ice,  usually  receive  the  same  projections,  which, 
when  their  plan  is  a  semi-ellipse,  must  be  somewhat  greater  than 
the  semi- width  of  the  pier.     Their  general  vertical  outline  is 


212 


BRIDGES,  ETC. 


columnar,  being  either  straight  or  swelled,  (Figs.  Ill,  112,  113, 
114.)    They  should  be  built  as  high  as  the  ordinary  highest 


Fig.  Ill — Represents  in  elevation  starlings  A,  their  hoods  B,  the  vonssoirs  C,  the  spandrels  D 
and  the  combination  of  their  courses  and  joints  with  each  other  in  an  oval  arch  of  thret 
centres. 

E,  parapet;  F,  cornice. 


]i\w\\\UMfi 


,n 


\ 

\ 

\ 

-1      '. 

54 


/        


A 


Fig.  113— Represents  In  elevation  the  combinations  of  the  same  elements  as  in  Fig.  Ill 
for  a  flat  segmental  arch. 


Fig.  115— Represents  in  elevation  the  combinations  of  the  same  elements  as  In  Fig.  112,  from  th« 

bridge  of  Neuilly,  atd  oval  of  eleven  centres. 
cm,  curve  of  intrados. 
on,  arc  of  circle  traced  c  n  the  head  of  the  bridge. 


STONE   BKIDGES. 


213 


i- 


_Jl 


Fig.  114 — Represents  a  cross  section 
and  elevation  through  the  crown 
of  Fig.  118,  showing  the  arrange- 
ment also  of  the  roadway,  foot- 
paths, parapet,  and  cornice. 


water-level.  They  are  finished  at  top  with  a  coping  stone  to 
preserve  the  masonry  from  the  action  of  rain,  &c. :  this  stone, 
termed  the  hood,  may  receive  a  conical,  a  spheroidal,  or  any 
other  shape  which  will  subserve  the  object  in  view,  and  produce 
a  pleasing  architectural  effect,  in  keeping  with  the  locality. 

In  streams  subject  to  freshets  and  ice,  the  up  stream  starlings 
should  receive  a  greater  projection  than  those  down  stream,  and, 
moreover,  be  built  in  the  form  of  an  inclined  plane  (Fig.  115) 


N 


N 


Fig.  115— Represents  a  side  elevation  It 
and  plan  N  of  a  pier  of  the  Potomac 
aqueduct,  arranged  with  an  ice- 
breaker starling. 

A,  up-stream  starling,  with  the  inclin- 
ed ice-breaker  D  which  rises  from 
the  low-water  level  above  that  of  the 
highest  freshets. 

B,  down-stream  starling. 

C,  face  of  pier. 

E,  top  ol  pier. 

F,  horizontal  projection  of  top  of  ice- 
breaker. 

QO,  horizontal  projection  of  faces  of 
pier  and  starlings. 


to  facilitate  the  breaking  of  the  ice,  and  its  passage  through 
the  arches. 
567.  Where  the  banks  of  a  water-course  spanned  by  a  bridge 


214 


BRIDGES,    ETC. 


are  so  steep  and  difficult  of  access  that  the  roadway  ir.ust  be 
raised  to  the  same  level  with  their  crests,  security  for  the  founda- 
tion, and  economy  in  the  construction  demand  that  hollow  or 
open  piers  be  used  instead  of  a  solid  mass  of  masonry.  A  con- 
struction of  this  kind  requires  great  precaution.  The  facing 
courses  of  the  piers  must  be  of  heavy  blocks  dressed  with  ex- 
treme accuracy.  The  starlings  must  be  built  solid.  The  faces 
must  be  connected  by  one  or  more  cross  tie-walls  of  heavy,  well- 
bonded  blocks  ;  the  tie-walk  being  connected  from  distance  to 
distance  vertically  by  strong  tie-blocks ;  or,  if  the  width  of  the 
pier  be  considerable,  by  a  tie-wall  along  its  centre  line. 

568.  The  foundations,  the  dimensions,  and  the  form  of  the 
abutments  of  a  bridge  will  be  regulated  upon  the  same  principles 
as  the  like  parts  of  other  arched  structures;  a  judicious  con- 
formity to  the  character  of  strength  demanded  by  the  structure, 
and  to  the  requirements  of  the  locality  being  observed.  The 
walls  which  at  the  extremities  of  the  bridge  form  the  con- 
tinuation of  the  heads,  and  sustain  the  embankments  of  the  ap- 
f>roaches, — and  which,  from  their  widening  out  from  the  general 
ine  of  the  heads,  so  as  to  form  a  gradual  contraction  of  the 
avenue  by  which  the  bridge  is  approached,  are  termed  the  wing- 
walls, — serve  as  firm  buttresses  to  the  abutments.  In  some  cases 
the  back  of  the  abutment  is  terminated  by  a  cylindrical  arch, 
(Fig.  116,)  placed  on  end,  or  having  its  right-line  elements  ver- 


Fig.  116 — Represents  a  horizontal  section  of 
an  abutment  A  with  curved  wing-walls  B, 
B,  connected  with  a  central  buttress  C  and  a 
cross  tie-wall  D. 


Fig.  117 — Represents  a  horizontal 
section  of  an  abutment  A  with 
straight  wing-walls  B,  B,  ter- 
minated by  return-Wills  0,  C. 

D,  central  buttress. 


tical,  which  connects  the  two  wing-walls.     In  others  (Fig.  117) 


STONE    BRIDGES. 


215 


a  rectangular-shaped  buttress  is  built  back  from  the  centre  line 
of  the  abutment,  and  is  connected  with  the  wing-walls  either  by 
horizontal  arches,  or  by  a  vertical  cross  tie-wall. 

569.  The  wing-walls  may  be  either  plane  surface  walls  (Fig. 
118)  arranged  to  make  a  given  angle  with  the  heads  of  the  bridge, 


Fig  115 — Represents  an  elevation  M  and  plan  N  of  a 
portion  of  a  single  arch  bridge  with  straight  wing- 
walls  sustaining  an  embankment  across  the  valley  of 
the  water-course. 

a,  a,  face  of  wing-wall. 

b,  b',  side  slope  of  embankment 

c,  c',  top  of  wing-wall. 

o,  o',  fender  or  guard  stones. 


or  they  may  be  curved  surface-walls  presenting  their  concavity, 
(Fig.  126,)  or  their  convexity  to  the  exterior  ;  or  of  any  other 
shape,  whether  presenting  a  continuous,  or  a  broken  surface,  that 
the  1<  tcality  may  demand.  Their  dimensions  and  form  of  profile 
will  be  regulated  like  those  of  any  other  sustaining  wall ;  and 
they  receive  a  suitable  finish  at  top  to  connect  them  with  the 
bridge,  and  make  them  conform  to  the  outline  of  the  embank- 
ments, or  other  approaches. 

570.  The  arches  of  bridges  demand  great  care  in  proportion- 
ing the  dimensions  of  the  voussoirs,  and  procuring  accuracy  in 
their  tonus,  as  the  strength  of  the  structure,  and  the  permanence 
of  its  figure,  will  chiefly  depend  upon  the  attention  bestowed  on 
these  points.  Peculiar  care  should  be  given  in  arranging  the 
masonry  above  the  piers  which  lies  between  the  two  adjacent 
arches.  In  some  of  the  more  recent  bridges,  (Fig.  120,)  this  part 
isbuilt  up  solid  but  a  short  distance  above  the  imposts,  generally 
not  higher  than  a  fourth  of  the  rise,  and  is  finished  with  a  reversed 


216 


BRIDGE8,  ETC. 


arch  to  give  greater  security  against  the  effects  of  the  pressure 
thrown  upon  it. 


Fig.  120 — Represents  a  longitudinal  section  of  a  portion  of  a  pier  and  foundations,  and  of  an  arch 
and  its  centre  of  the  new  London  bridge  over  the  Thames. 

A,  finish  of  solid  spandrel  with  reversed  arch. 

B,  wedge  <>f  striking  plates. 

C,  recess  over  the  starlings  for  seats 

The  backs  of  the  arches  should  be  covered  with  a  water-tight 
capping  of  beton,  and  a  coating  of  asphaltum. 

i>'{\.  The  entire  spandrel  courses  of  the  heads  are  usually  not 
laid  until  the  arches  have  been  uncentred,  and  have  settled,  in 
order  that  the  joints  of  these  courses  may  not  be  subject  to  any 
other  cause  of  displacement  than  what  may  arise  from  the  effects 
of  variations  of  temperature  upon  the  arches.     The  thickness  of 


STONE  BRIDGES. 


217 


the  head-walls  will  depend  upon  the  method  adopted  for  support- 
ing the  roadway.  If  this  be  by  a  filling  of  earth  between  the 
head-walls,  then  their  thickness  must  be  calculated  not  only  to 
resist  the  pressure  of  the  earth  which  they  sustain,  but  allowance 
must  also  be  made  for  the  effects  of  the  shocks  of  floating  bodies 
in  weakening  the  bond,  and  separating  the  blocks  from  their  mor- 
tar-bed. The  more  approved  methods  of  supporting  the  roadway, 
and  which  are  now  generally  practised,  except  for  very  flat  seg- 
ment arches,  are  to  lay  the  road  materials  either  upon  broad  flag- 
ging stones  (Fig.  120, 121,)  which  rest  upon  thin  brick  walls  built 


Fig.  121— Represents  a  profile  of 
Fig.  120  through  the  centre  of  the 
pier,  showing  the  arrangement  of 
the  roadway  and  its  drainage,  <fcc. 

A,  section  of  masonry  of  pier  and 
spandrel. 

b,  &,  sections  of  walls  parallel  to  head- 
wall,  which  support  the  flagging 
stone  on  which  the  roadway  is 
laid. 

c,  section  of  head-wall  and  buttress 
above  the  starling  d. 

e,  footpath. 

/,  recess  for  seats  over  the  buttress. 

o,  cornice  and  parapet. 

It,  vertical  conduit  in  the  pier  com- 
municating with  two  others  under 
the  roadway  from  the  side  chan- 
nels. 


parallel  to  the  head-walls,  and  supported  by  the  piers  and  arches; 
or  by  small  arches,  (Fig.  122,)  for  which  these  walls  serve  as 
piers ;  or  by  a  system  of  small  groined  arches  supported  by 
pillars  resting  upon  the  piers  and  main  arches.  When  either 
of  these  methods  is  used,  the  head-walls  may  receive  a  mean 
thickness  of  one  fifth  of  their  height  above  the  solid  spandrel. 

572.  Superstructure.  The  superstructure  of  a  bridge  consists 
of  a  cornice,  the  roadway  and  footpaths,  &c,  and  a  parapet. 

The  object  of  the  cornice  is  to  shelter  the  face  of  the  head- 
walls  from  rain.  To  subserve  this  purpose,  its  projection  beyond 
the  surface  to  be  sheltered  should  be  the  greater  as  the  altitude 
of  the  sheltered  part  is  the  more  considerable.  This  rule  will 
require  a  cornice  with  supporting  blocks,  (Fig.  123,)  termed 
mod  ill  tons,  below  it,  whenever  the  projecting  part  would  be 
actually,  or  might  seem  insecure  from  its  weight.  The  height 
of  the  cornice,  including  its  supports,  should  generally  be  equal 
to  its  projections;  this  will  often  require  more  or  less  of  detail 
in  the  profile  of  the  cornice,  in  order  that  it  may  not  appear 
heavv.     The  top  surface  of  the  cornice  should  be  a  little  above 

28 


218 


BRIDGES,  ETC. 


that  of  the  footpath,  or  roadway,  and  be  slightly  sloped  out- 
ward ;  the  bottom  should  be  arranged  with  a  situable  larmier, 


Fig.  122— Represents  a  section  through  the  axis  of  a  pier  of  bridge  huilt  of  stone  with  brick 
filling,  showing  the  arrangement  for  supporting  the  roadway  on  small  arches. 

or  drip,  to  prevent  the  water  from  finding  a  passage  along  ite 
under  surface  to  the  face  of  the  wall. 


Fig.  128 — Represents  a  section  through  the  crown  of 
an  arch,  showing  the  cornice  a,  modillion  b,  para- 
pet c,  and  footpath  d. 

A,  key-stones. 

B,  side  elevation  of  soffit 


5T3.  The  parapet  surmounts  the  cornice,  and  should  be  high 
enough  to  secure  vehicles  and  foot-passengers  from  accidents, 
without  however  intercepting  the  view  from  the  bridge.  The 
parapet  is  usually  a  plain  low  wall  of  cut  stone,  surmounted  by 
a  copine  slightly  rounded  on  its  top  surface.     In  bridges  which 


STONE   BEIDGE8.  219 

have  a  character  of  lightness,  like  those  with  flat  segment  arches, 
the  parapet  may  consist  of  alternate  panels  of  plain  wall  and 
balustrades,  provided  this  arrangement  be  otherwise  in  keeping 
with  the  locality.  The  exterior  face  of  the  parapet  should  not 
project  beyond  that  of  the  heads.  The  blocks  of  which  it  is 
formed,  and  particularly  those  of  the  coping,  should  be  firmly 
secured  with  copper  or  iron  cramps. 

574.  The  width  of  the  roadway  and  of  the  footpaths  will  be 
regulated  by  the  locality ;  being  greatest  where  the  thoroughfares 
connected  by  the  bridge  are  most  frequented.  They  are  made 
either  of  broken,  or  of  pavingstone.  They  should  be  so  arranged 
that  the  surface-water  from  rain  shall  run  quickly  into  the  side 
channels  left  to  receive  it,  and  be  conducted  from  thence  by  pipes 
which  lead  to  vertical  conduits  (Fig.  121)  in  the  piers  that  have 
their  outlets  in  one  of  the  faces  of  the  piers,  and  below  the 
lowest  water-level. 

575.  Strong  and  durable  stone,  dressed  with  the  chisel,  or 
hammer,  should  alone  be  used  for  the  masonry  of  bridges  where 
the  span  of  the  arch  exceeds  fifty  feet.  The  interior  of  the 
piers,  and  the  backing  of  the  abutments  and  head-walls  may,  for 
economy,  be  of  good  rubble,  provided  great  attention  be  bestowed 
upon  the  bond  and  workmanship.  For  medium  and  small  spans 
a  mixed  masonry  of  dressed  stone  and  rubble,  or  brick,  may  be 
used  ;  and,  in  some  cases,  brick  alone.  In  all  these  cases  (Figs. 
122,  124)  the  starlings, — the  foundation  courses, — the  impost 
stone, — the  ring  courses,  at  least  of  the  heads, — and  the  key- 
stone, should  be  of  good  dressed  stone.  The  remainder  may  be 
of  coursed  rubble,  or  of  the  best  brick,  for  the  facing,  with  good 
rubble  or  brick  for  the  fillings  and  backings.  In  a  mixed  masonry 
of  this  character  the  courses  of  dressed  stone  may  project  slight- 
ly beyond  the  surfaces  of  the  rest  of  the  structure.  The  archi- 
tectural effect  of  this  arrangement  is  in  some  degree  pleasing, 
particularly  when  the  joints  are  chamfered  ;  and  the  method  is 
obviously  useful  in  structures  of  this  kind,  as  protection  is  af- 
forded by  it  to  the  surfaces  which,  from  the  nature  of  the  mate- 
rial, or  the  character  of  the  work,  offer  the  least  resistance  to  the 
destructive  action  of  floating  bodies.  Hydraulic  mortar  should 
alone  be  used  in  every  part  of  the  masonry  of  bridges. 

57'!.  Approaches.  The  arrangement  of  the  approaches  will 
depend  upon  the  number  and  direction  of  the  avenues  leading  to 
the  bridge, — the  width  of  the  avenues,  and  their  position  above 
or  below  the  natural  surface  of  the  ground, — and  the  locality. 
The  principal  points  to  be  kept  in  view  in  their  arrangement  are 
to  procure  an  easy  and  safe  access  to  the  bridge  for  vehicles,  and 
not  to  obstruct  unnecessarily  the  channels,  for  purposes  of  navi- 
gation, which  may  be  requisite  under  the  extreme  arches. 


22C 


BRIDGE?,    ETC. 


i    "    i        ,    '     .        ii     ~T^  '.'ii'        i 
i        !        I        !        ■        ii        I       i        i  — r       i     t- 
111'        I        I        i   _  i        ;        '       !    .     ■.    '  ; 


•y  f   t   I 


Fig.  124 — Ri-p'-fgents  an  elevation  of  a  pier,  a  portion  of  two  arches,  and  the  centre  of  the  bridge 
of  which  Fig.  122  is  the  section. 

A,  face  of  starling. 

B,  hood. 

C,  voussoirs  with  chamfered  joints. 

When  the  avenue  to  the  bridge  is,  by  an  embankment,  in  the 
6ame  line  as  its  axis,  and  the  roadway  and  bridge  are  of  the  same 
width,  the  head-walls  of  the  bridge  (Fig.  125)  may  be  prolonged 
sufficiently  far  to  allow  the  foot  of  the  embankment  slope  to  fall 
within  a  few  feet  of  the  crest  of  the  slope  of  the  water-course  ; 
this  portion  of  the  embankment  slope  being  shaped  into  the  form 
of  a  quarter  of  a  cone,  and  reveted  with  dry  stone  or  sods,  to  pre- 
serve its  surface  from  the  action  of  rain. 

When  several  avenues  meet  at  a  bridge,  or  where  the  width 
of  the  roadway  of  a  direct  avenue  is  greater  than  that  of  the 


STONE    BRIDGES. 


221 


bridge,  the  approaches  are  made  by  gradually  widening  the  out 
let  from  the  bridge,  until  it  attains  tne  requisite  width,  by  means  ol 


125 — Elevation  If  and  plan  N,  showing  the  manner  of  arranging  the  embankments  of  tho 
approaches,  when  the  head-walls  of  the  bridge  are  simply  prolonged. 

,  side  slope  of  embankment. 

,  dry  stone  facing  of  the  embankment  where  its  end  is  rounded  off,  forming  a  quarter  of  a  cone 

finish. 

,  flight  of  steps  for  foot-passengers  to  ascend"the  embankment. 

,  embankment  arranged  as  above,  but  simply  sodded. 

,  feeing  of  dry  stone  for  the  side  slopes  of  the  banks. 
e,  6',  facing  of  *he  bottom  of  the  stream. 

wing-walls  of  any  of  the  usual  forms  that  may  suit  the  locality. 
The  form  of  wing-wall  (Fig.  126)  presenting  a  concave  surface 
outward  is  usually  preferred  when  suited  to  the  locality,  both 


Fig. 

a,  a 
6,6 

0.  c\ 
d,d 


Fig.  126 — Represent  an  elevation  M  and  plan  N 
of  a  curved  face  wing-wall. 

A,  front  view  of  wing-wall. 

B,  B',  slope  of  embankment. 


fcr  its  architectural  effect  and  its  strength.  When  made  ot 
dressed  stone  it  is  of  more  difficult  construction  and  more  ex- 
pensive than  the  plane  surface  wall. 


222  BRIDGES,   ETC.     ■ 

In  order  that  tlie  approaches  may  not  obstruct  the  com- 
munications along  the  banks  for  the  purposes  of  navigath  n,  an 
arched  passage-way  will,  in  most  cases,  be  requisite  under  the 
roadway  of  the  approach  and  behind  the  abutment  of  the  ex- 
treme arch,  for  horses,  and,  if  necessary,  vehicles.  When  the 
form  of  the  arch  will  admit  of  it,  as  in  flat  segment  arches,  a 
roadway,  projecting  beyond  the  face  of  the  abutment,  may  be 
made  under  the  arch  for  the  same  purpose. 

577.  Water-vnngs.  To  secure  the  natural  banks  near  the 
bridge,  and  the  foundations  of  the  abutments  from  the  action  of 
the  current,-  a  facing  of  dry  stone,  or  of  masonry,  should  be  laid 
upon  the  slope  of  the  banks,  which  should  be  properly  prepared 
to  receive  it,  and  the  foot  of  the  facing  must  be  secured  by  a 
mass  of  loose  stone  blocks  spread  over  the  bed  around  it,  in  ad- 
dition to  which  a  line  of  square-jointed  piles  may  be  previously 
driven  along  the  foot.  When  the  face  of  the  abutment  projects 
beyond  the  natural  banks,  an  embankment  faced  with  stone 
should  be  formed  connecting  the  face  with  points  on  the  natural 
banks  above  and  below  the  bridge.  By  this  arrangement, 
termed  the  water-wings,  the  natural  water-way  will  be  gradu- 
ally contracted  to  conform  to  that  left  by  the  bridge. 

578.  Enlargement  of  ^Yater-way.  In  the  full  centre  and  oval 
arches,  when  the  springing  lines  are  placed  low,  the  spandrels 
present  a  considerable  surface  and  obstruction  to  the  current 
during  the  higher  stages  of  the  water.  This  not  only  endangers 
the  safety  of  the  bridge,  by  the  accumulation  of  drift-wood  and 
ice  which  it  occasions,  but,  during  these  epochs,  gives  a  heavy 
appearance  to  the  structure.  To  remedy  these  defects  the  solid 
angle,  formed  by  the  heads  and  the  soffit  of  the  arch,  may  be 
truncated,  the  base  of  the  cuneiform-shaped  mass  taken  away 
being  near  the  springing  lines  of  the  arch,  and  its  apex  near  the 
crown.  The  form  of  the  detached  mass  may  be  variously  ar- 
ranged. In  the  bridge  of  Nenilly,  which  is  one  of  the  first  where 
this  expedient  was  resorted  to,  the  surface,  marked  F,  (Figs.  113, 
114,)  left  by  detaching  the  mass  in  question,  is  warped,  and  lies 
between  two  plane  curves,  the  one  an  arc  of  a  circle  n  o,  traced 
on  the  head  of  the  bridge,  the  other  an  oval  m  o  op,  traced  on 
the  soffit  of  the  arch.  This  affords  a  funnel-shaped  water-way 
to  each  arch,  and,  during  high  water,  gives  a  light  appearance 
to  the  structure,  as  the  voussoirs  of  the  head  ring-course  have 
then  the  appearance  of  belonging  to  a  flat  segmental  arch. 

579.  Centres.  The  framing  of  centres,  ana  the  arrangement 
for  striking  them,  having  been  already  fully  explained  under  the 
article  Framing,  with  illustrations  taken  from  some  of  the  most 
celebrated  recent  structures,  nothing  further  need  be  here  added 
than  to  point  out  the  necessity  of  great  care  both  in  the  combi  • 


•    STONE   BRIDGE8.  223 

aation  of  the  frame,  and  in  its  mechanical  execution,  in  order 
to  prevent  any  change  in  the  form  of  the  arch  while  under  con- 
struction. The  English  engineers  have  generally  been  more 
successful  in  this  respect  than  the  French.  The  latter,  in  several 
of  their  finest  bridges,  used  a  form  of  centre  composed  of  seve 
ral  polygonal  frames,  with  short  sides,  so  inscribed  within  each 
other  that  the  angles  of  the  one  corresponded  to  the  middle  of 
the  sides  of  the  other.  The  sides  of  each  frame  were  united  by 
joints,  and  the  series  of  frames  secured  in  their  respective  posi 
tions  by  radial  pieces,  in  pairs,  notched  upon  and  bolted  to  the 
frames,  which  they  clamped  between  them.  A  combination  of 
this  character  can  preserve  its  form  only  under  an  equable 
pressure  distributed  over  the  back  of  the  exterior  polygon. 
When  applied  to  the  ordinary  circumstances  attending  the  con- 
struction of  an  arch,  it  is  found  to  undergo  successive  changes 
of  shape,  as  the  voussoirs  are  laid  on  it ;  rising  first  at  the  crown, 
then  yielding  at  the  same  point  when  the  key -stone  and  the  ad- 
jacent voussoirs  are  laid  on.  The  English  engineers  have  gen- 
erally selected  those  combinations  in  which,  the  pressures  being 
transmitted  directly  to  fixed  points  of  support,  no  change  of 
form  can  take  place  in  the  centre  but  what  arises  from  the 
contraction  or  elongation  of  the  parts  of  the  frame. 

580.  General  Remarks.  The  architecture  of  stone  bridges  has, 
within  a  somewhat  recent  period,  been  carried  to  a  very  high  de- 

free  of  perfection,  both  in  design  and  in  mechanical  execution, 
ranee,  in  this  respect. lias  givenan  example  to  the  world,  and  has 
found  worthy  rivals  in  the  rest  of  Europe,  and  particularly  in 
Great  Britain.  Her  territory  is  dotted  over  with  innumerable 
fine  monuments  of  this  character,  which  attest  her  solicitude  as 
well  for  the  public  welfare  as  for  the  advancement  of  the  in- 
dustrial and  liberal  arts.  For  her  progress  in  this  branch  of 
architecture,  France  is  mainly  indebted  to  her  School  and  her 
Corps  of  Points  et  Chaussees:  institutions  which,  from  the  time 
of  her  celebrated  engineer  Perronet,  have  supplied  her  with  a 
long  line  of  names,  alike  eminent  in  the  sciences  and  art-  which 
pertain  to  the  profession  of  the  engineer. 

England,  although  on  somepoints  of  mechanical  skill  pertain- 
ing to  the  engineer's  art  the  superior  of  France,  holds  the  second 
rank  to  her  in  the  science  of  her  engineers.  Without  establish- 
ments for  professional  training  corresponding  to  those  of  France, 
the  English  engineers,  as  a  body,  have,  until  within  a  few  years, 
labored  under  the  disadvantage  of  having  none  of  those  institu- 
tions which,  by  creating  a  common  bond  of  union,  serve  not  only 
to  diffuse  science  throughout  the  whole  body,  but  to  raise  merit 
to  its  proper  level,  and  frown  down  alike,  through  an  enlightened 
esprit  de  co?^ps,  the  assumptions  of  ignorant  pretension,  and  the 


224  BRIDGES,  ETC.     . 

malevolence  of  petty  jealousies.  Although,  as  a  body,  less  ad 
vantageously  placed,  in  these  respects,  than  theirmore  thorough- 
bred brethren  of  France,  the  engineers  of  England  can  point, 
with  a  just  feeling  of  pride,  not  only  to  the  monuments  of  their 
skill,  but  to  individual  names  among  them  which,  achieved 
under  the  peculiar  obstacles  ever  attendant  upon  self-education, 
yet  stand  in  the  first  rank  of  those  by  whose  genius  the  industrial 
arts  have  been  advanced  and  ennobled. 

The  other  European  States  have  also  contributed  largely  to 
bridge  architecture,  although  their  efforts  in  this  line  are  less 
widely  known  through  their  publications  than  those  of  France 
andEngland.  Amongthemany  bridges  belonging  to  Italy,  may 
be  justly  cited  the  far-famed  Rialto  /  the  bridge  of  Santa  Trinita 
at  Florence,  the  curve  of  whose  intrados  was"  so  long  a  mathe- 
matical puzzle;  and  the  recent  single  arch  over  the  Dora  Hiparia 
near  Turin. 

In  the  United  States,  the  pressing  immediate  wants  of  a  young 
people,  who  are  still  without  that  accumulated  capital  by  which 
alone  great  and  lasting  public  monuments  can  be  raised,  have  pre- 
vented much  being  done,  in  bridge  building,  except  of  a  temporary 
character.  The  bridges,  viaducts,  and  aqueducts  of  stone  in  our 
country,  almost  without  an  exception,  have  been  built  of  rustic 
work  through  economical  considerations.  The  selection  of  tin's 
kind  of  masonry,independently  of  its  cheapness,  has  the  merit  of  ap- 
propriateness, when  taken  in  connection  with  the  natural  features 
of  the  localities  where  most  of  the  sestructures  are  placed.  Among 
the  works  of  this  class,  may  be  cited  the  railroad  bridge,  called  the 
Thomas  Viaduct,  over  the  Patapsco,  on  the  line  of  the  Baltimore 
and  Washington  railroad,  designed  and  builtby  Mr.  B.  H.Latrobe, 
the  engineer  of  the  road.  This  is  one  of  the  few  existing  bridge 
structures  with  a  curved  axis.  The  engineer  has  very  happily 
met  the  double  difficulty  before  him,  of  being  obliged  to  adopt  a 
curved  axis,  and  of  the  want  of  workmen  sufficiently  conversant 
with  the  application  of  working  drawings  of  a  rather  compli- 
cated character,  by  placing  full  centre  cylindrical  arches  upon 
■piers  with  a  trapezoidal  horizontal  section.  This  structure,  with 
me  exception  of  some  minor  details  in  rather  questionable  taste, 
as  the  slight  iron  parapet  railing,  for  example,  presents  an  impo- 
sing aspect,  and  does  great  credit  to  the  intelligence  and  skill  of 
the  engineer,  at  the  time  of  its  construction,  but  recently  launched 
in  a  new  career.  The  fine  single  arch,  known  as  the  CarroUon 
Viaduct,  on  the  Baltimore  and  Ohio  railroad,  is  also  highly  credit- 
able to  the  science  and  skill  of  the  engineer  and  mechanics  under 
whom  it  was  raisd.  One  of  the  largest  bridges  in  the  United 
States,  designed  and  partly  executed  in  stone,  is  the  Potomac 
Aqueduct  at  Georgetown,  where  the  Chesapeake  and  Ohio  canal 


STONE  BRIDGES. 


225 


intersects  the  Potomac  river.  This  work,  to  which  a  wooden 
superstructure  has  been  made,  was  built  under  the  superintend- 
ence of  Captain  Turnbull  of  the  U.  S.  Topographical  Engineers. 
In  the  published  narrative  of  the  progress  of  this  work,  a  very  full 
account  is  given  of  all  the  operations,  in  which,  while  the  re- 
sources and  skill  of  the  engineer,  in  a  very  difficult  and,  to  him, 
untried  application  of  his  art,  are  left  to  be  gathered  by  the  reader 
from  the  successful  termination  of  the  undertaking,  his  failures 
are  stated  with  a  candor  alike  creditable  to  the  man,  and  worthy 
of  imitation  by  every  engineer  who  prizes  the  advancement  of  his 
art  above  that  personal  reputation  which  a  less  truthful  course 
may  place  in  prospect  before  him. 

581.  The  following  table  contains  a  summary  of  the  principal 
details  of  some  t5f  the  more  noted  stone  bridges  of  Europe. 


NAME  Or  BRIDGE. 

River. 

Form  of 

Numb, 
of 

Span  of           Depth 
widest  [Rise,  of  key- 

Width  I 
be'-«Da,e. 

Name  of  engineer 

arches. 

span. 

stone. 

heads. 

Vieille-Brionde    . 

Allier. 

Segment. 

1 

178 

69 

5.3 

1 
-      1454 

Grenier&Estone. 

Rialto     .... 

— 

" 

1 

98.G 

23 

- 

-      1578 

Michel  Angelo. 

Claix      .... 

Drac. 

" 

1 

150 

54 

3.1 

-      1611 

— 

Neuilly  (A)     .    . 

Seine. 

Elliptical. 

5 

127.9 

31.9     5.3 

47.9    1774 

Perronet. 

Lavaur  .... 

Agout. 

" 

1 

160.5 

65    1  10.9 

-      1775 

Saget. 

Saint- .Maxence(B) 

Oise. 

Segment. 

3 

70.7 

6       5 

41.5    1784 

Perronet. 

Gignac    .... 

Erault 

Elliptical. 

1 

160 

44       6.5 

-      1793 

Garipuy. 

Jena  (C)     ... 

Seine. 

Segment. 

5 

91.8 

10.8     4.6 

43.7    1811 

Lamande. 

Rouen    .... 

Seine. 

" 

5 

101.7 

13.7     4.0 

49.2   1813 

Lamande. 

Waterloo  (D) .    . 

Thames. 

Elliptical. 

9 

120 

35    1    4.9 

45      1816 

Rennie. 

Gloucester  (E)    . 

Severn. 

" 

1 

150 

54        4.5 

35      1827 

Telford. 

London  (F)     .    . 

Thames. 

" 

5 

152 

:i7.rt    .-> 

56      1831 

Rennie. 

Turin  (G)   .    .    . 

Dora  Riparia 

Segment. 

1 

147.6 

18 

4.9 

40     1    - 

Mosca. 

Grosvenor  (H)     . 

Doe. 

1 

200 

42 

4 

-    ia33 

Hartley. 

(A.)  This  fine  structure,  designed  and  built  by  the  celebrated 
Perronet,  forms  an  epoch  in  bridge  architecture,  from  the  bold- 
ness of  its  design,  its  skilful  mechanical  execution,  and  the  simple 
but  appropriate  character  of  its  architectural  details.  The  curve 
of  the  intrados  is  an  oval  of  eleven  centres,  the  radius  of  the  arc 
at  the  spring  being  20.9  feet,  and  that  of  the  arc  at  the  crown 
159.1  feet.  The  engineer  conceived  the  idea  of  giving  to  the 
soffit  a  funnel  shape,  by  widening  it  at  the  heads,  from  the  crown 
to  the  springing  line.  This  he  effected  by  connecting  the  soffit 
of  each  arch  and  the  heads  by  a  warped  surface,  which  passed, 
on  the  one  hand,  through  a  fiat  circular  arc,  described  upon  the 
heads  through  the  points  of  the  crown  and  the  top  of  the  two  ad- 
jacent starlings,  and,  on  the  other,  through  two  curves  on  the 
soffit,  cut  out  by  two  vertical  planes,  obliqu6  to  the  axis,  passed 
through  the  highest  point  of  the  curve  on  the  heads,  and  through 
points  on  the  two  respective  springing  lines  of  the  arch.  The  ob 
ject  of  this  arrangement  was  twofold ;  first, — as  the  springing  lines 
were  placed  at  the  low-water  level,  the  bridge,  during  the  seasoni 

29 


226  BRIDGES,  ETC. 

of  high  water,  would  have  appeared  rather  heavy,  as  the  greater 
part  of  the  soffit,  at  this  period,  would  have  been  under  water, — 
it  gave  the  bridge  a  lighter  appearance  during  the  epochs  of  high 
water ;  and,  second,  as  the  obstruction  to  the  free  flow  of  the 
water  from  the  spandrels  would  be  very  considerable  at  the  same 
periods,  the  funnel  form  given  to  the  soffit  at  the  heads  partially 
remedied  this  inconvenience. 

The  axis  of  the  roadway,  the  cornice,  and  all  the  correspond- 
ing architectural  lines  were  made  horizontal,  a  feature  in  bridge 
architecture  which  the  reputation  of  Perronet  has  since  rendered 
classical ;  and  to  obtain  which  points  truly  essential  conditions 
have  in  some  more  recent  structures  been  sacrificed. 

The  abutments  are  32  feet  thick  at  the  springing  lines,  and  the 
piers  but  13.8  feet  at  the  same  point,  giving  an  -example  of  judi- 
cious boldness  combined  with  adequate  strength,  on  scientific 
principles,  which  had  been  partially  lost  sight  of  by  preceding  en- 
gineers in  designing  this  part  of  bridges. 

The  centres  of  the  Neuilly  bridge  were  designed  upon  the 
faulty  principle  of  concentric  polygonal  frames.  Perronet  was 
aware  of  the  inconveniences  of  this  combination,  and  in  no  part 
of  the  construction  of  the  bridge  than  in  this  was  more  sagacious 
forethought  displayed  by  him,  in  providing  for  foreseen  contingen- 
cies, nor  greater  resources  and  skill  in  remedying  those  which 
could  not  have  been  anticipated.  An  oversight,  rather  more 
serious  in  its  consequences,  was  committed  in  widening  the  natu- 
ral water-way  of  the  river  where  the  bridge  was  erected ;  the 
effect  of  this  has  been  a  gradual  deposition  near  the  bridge,  and 
an  obstruction  of  the  navigable  channels. 

The  bridge  of  Neuilly  is  a  noble  monument  of  the  genius  and 
practical  skill  of  its  engineer.  The  style  of  its  architecture,  both 
as  a  whole  and  in  its  several  parts,  is  imposing  and  in  the  best 
taste. 

(B)  This  bridge  was  built  after  the  designs  of  Perronet.  Se- 
duced by  a  thorough  knowledge  of  the  capabilities  of  his  art,  the 
engineer  was  led,  in  planning  this  structure,  into  the  error  of 
sacrificing  apparent  strength,  for  the  purpose  of  producing  great 
boldness  and  lightness  of  design.  This  he  effected  by  placing 
very  flat  segment  arches  upon  piers  formed  of  four  columns  ;  the 
two,  forming  the  starlings,  being  united  to  the  two  adjacent  by  a 
connecting  wall,  an  interval  being  left  between  the  two  centre 
columns.  The  diameters  of  the  columns  are  9.6  feet,  with  the 
same  interval  between  them. 

The  engineer  who  constructed  the  bridge,  apprehensive  appa- 
rently for  its  safety,  introduced  into  the  courses  of  the  piers  and 
of  the  arches  a  large  quantity  of  iron  ties  and  cramping  pieces,  a 
measure  of  orecaution  which,  if  necessary,  ought  to  have  con- 


STONE  BRIDGES.  227 

3emned  the  original  designs,  although  supported  by  the  high 
authority  of  Perronet,  and  caused  others  to  be  substituted  for 
them. 

(C)  This  bridge,  now  designated  as  the  Pont  de  VEcole  Mili 
taire,  from  its  locality,  and  the  bridge  of  Rouen,  are  built  upon 
nearly  the  same  designs.  The  former  is  a  model  of  architectural 
taste  and  of  skilful  workmanship.  Its  horizontal  architectural 
lines,  its  fine  cornice,  copied  from  that  of  the  temple  of  Mars  the 
Avenger,  and  the  sculptured  wreath  on  its  spandrels,  form  a 
whole  of  singular  beauty. 

(D)  This  bridge,  designated  when  first  built  as  the  Strand 
Bridge,  is  worthy  of  the  great  metropolis  in  which  it  is  placed. 
The  engineer,  influenced  perhaps  by  other  examples  of  the  same 
character  in  the  vicinity  of  this  structure,  has  placed  small  col- 
umns upon  the  starlings,  which  support  recesses  with  seats  for 
foot-passengers,  and  has  thus,  in  no  inconsiderable  degree,  de- 
prived the  bridge  of  that  imposing  character  which  its  massive- 
ness,  and  the  excellent  material  of  which  it  is  built,  could  not 
otherwise  have  failed  to  produce. 

(E)  This  fine  elliptical  arch  is,  in  some  respects,  built  in  imi- 
tation of  the  Neuilly  bridge,  writh  a  funnel-shaped  soffit.  Its  gen- 
eral architectural  effect  is  heavy,  and  its  mere  ornamental  parts 
are  in  questionable  taste.  The  details  of  its  construction  are 
alike  monuments  of  the  eminent  professional  skill,  and  of  the 
truthfulness  of  character  of  the  great  engineer  who  planned  and 
superintended  it.  In  his  narrative  of  the  work,  Mr.  Telford  takes 
blame  to  himself  for  oversights  and  unanticipated  results,  in  which 
the  scrupulous  care  that  he  conscientiously  brought  to  every  un- 
dertaking committed  to  him  is  unwittingly  thrown  into  bolder 
relief,  by  the  very  confession  of  his  failures  ;  and  a  lesson  of  in- 
struction is  conveyed,  more  pregnant  with  important  consequences 
to  the  advancement  of  his  profession  than  the  recording  of  hun- 
dreds of  successful  instances  only  could  have  furnished. 

(F)  This  noble  work  of  Sir  John  Rennie  must  ever  rank  among 
the  master-pieces  of  bridge  architecture,  in  every  point  by  which 
this  class  of  structures  should  be  distinguished.  For  boldness, 
strength,  simplicity,  massiveness  without  heaviness,  and  a  happy 
adaptation  of  design  to  the  locality,  it  stands  unrivalled.  The 
beauty  which  is  generally  recognised  in  a  level  bridge  has,  in 
this,  been  judiciously  sacrificed  to  a  well-judged  economy ;  and 
the  artificial  approaches  have  thus  been  accommodated  to  the 
existing,  by  decreasing  the  dimensions  of  the  arches  from  the 
centre  to  the  two  extremities.  The  square  plain  buttresses, 
which  rise  above  the  starlings  and  support  the  recesses  for  seats, 
are  of  farther  obvious  utility  in  strengthening  the  head-walls, 
which,  at  these  points  are  of  considerable  height ;  and  they  alac 


228  BRIDGES.,  ETC. 

prodi  ce,  in  this  case,  a  not  unpleasing  architei  tural  effect,  in 
separating  the  unequal  arches,  without  impairing  the  unity  of  the 
general  design. 

(G)  This  is  the  boldest  single  arch  of  stone  now  standing,  and 
is  a  splendid  example  of  architectural  design  and  skilful  workman- 
ship. The  soffit  of  the  arch  is  made  slightly  funnel-shaped,  which 
gives  the  bridge  an  air  of  almost  too  great  boldness.  The  cornice, 
which  is  copied  from  the  same  model  as  that  of  the  bridge  of 
Jena ;  the  convex  cylindrical-shaped  wing-walls,  which  give  an 
approach  of  144  feet  between  the  parapets  ;  with  the  other  archi- 
tectural accessories,  have  made  this  bridge  a  model  of  good  taste 
for  imitation  under  like  circumstances.  From  the  omission  of  a 
usual  architectural  member,  there  is  perhaps  a  slight  feeling  of 
nakedness  produced  on  the  mind  of  the  rigid  connoisseur  in  art, 
on  first  seeing  this  structure,  and  its  beauty  is  in  some  degree 
marred  by  this  want. 

The  abutments  of  this  bridge  are  40  feet  thick  at  the  founda- 
tions, and,  besides  the  wing-walls,  are  strengthened  by  two  coun- 
terforts 20  feet  long  and  10  feel  wide. 

(H)  The  span  of  this  arch  is  the  widest  on  record.  For 
architectural  effect  this  bridge  presents  but  little  to  the  eye  that 
is  commendable  ;  for  this  the  engineer  who  superintended  it  is 
hardly  responsible,  except  so  far  as,  from  professional  sympathy 
and  respect  for  a  deceased  member  of  the  profession,  he  was  led 
to  adopt  the  designs  of  another.  The  abutments  form  a  continua- 
tion of  the  arch  ;  and  the  other  details  of  the  construction  through- 
out exhibit  that  thorough  acquaintance  with  their  art  for  which 
the  Hartleys,  father  and  s^n,  are  well  known  to  the  profession. 

582.  The  practice  of  bnage  building  is  now  generally  the  same 
throughout  the  civilized  world.  In  France,  the  method  of  laying 
the  foundations  by  caissons  has,  in  most  of  their  later  works,  been 
preferred  by  her  engineers  to  that  of  coffer-dams  ;  and  in  the  su- 
perstructure of  their  bridges  the  French  engineers  have  generally 
filled  in,  between  the  arches  and  the  roadway,  with  solid  material. 
In  some  of  these  bridges,  as  in  that  of  Bordeaux,  where  appre- 
hension was  felt  for  the  stability  of  the  piling,  a  mixed  masonry 
of  stone  and  brick  was  used,  and  the  roadway  was  supported  by 
a  system  of  light-groined  arches  of  brick.  Among  the  recent 
French  bridges,  presenting  some  interesting  features  in  their  con- 
struction, may  be  cited  that  of  Souillac  over  the  Dordogne.  The 
river  at  this  place  having  a  torrent-like  character,  and  the  bed 
being  of  lime-stone  rock  with  a  very  uneven  surface,  and  occa 
skmal  deep  fissures  filled  with  sand  and  gravel,  the  obstacle  tc 
using  either  the  caisson,  or  the  ordinary  coffer-dam  for  the  foun- 
dations, was  very  great.  The  engineer,  M.  Yicat,  so  well  known 
»y  his  researches  upon  mortar,  &c,  devised,  to  obviate  these 


STONE  BRIDGES.  221 

difficulties,  the  plan  of  enclosing  the  area  of  each  pier  by  a  coffer* 
work  accurately  fitted  to  the  surface  of  the  bed,  and  of  filling  this 
with  beton  to  form  a  bed  for  the  foundation  courses.  This  he 
effected,  by  first  forming  a  frame-work  of  heavy  timber,  so  ar- 
ranged that  thick  sheeting-piles  could  be  driven  close  to  the  bot- 
tom, between  its  horizontal  pieces,  and  form  a  well-jointed  vessel 
to  contain  the  semi-fluid  material  for  the  bed.  After  this  coffer- 
work  was  placed,  the  loose  sand  and  gravel  was  scooped  from 
the  bottom,  the  asperities  of  the  surface  levelled,  and  the  fissures 
were  voided,  and  refilled  with  fragments  of  a  soft  stone,  which  it 
was  found  could  be  more  compactly  settled,  by  ramming,  in  the 
fissures,  than  a  looser  and  rounder  material  like  gravel.  On  this 
prepared  surface,  the  bed  of  beton,  which  was  from  12  to  15  feet 
in  thickness,  was  gradually  raised,  by  successive  layers,  to  with- 
in a  few  feet  of  the  low-water  level,  and  the  stone  superstructure 
then  laid  upon  it,  by  using  an  ordinary  coffer-dam  that  rested  on 
the  frame-work  around  the  bed.  In  this  bridge,  as  in  that  of 
Bordeaux,  a  provisional  trial-weight,  greater  than  the  permanent 
load,  was  laid  upon  the  bed,  before  commencing  the  superstruc- 
ture. 

To  give  greatc:  security  to  their  foundations,  the  French  usually 
surround  them  with  a  mass  of  loose  stone  blocks  thrown  in  and 
allowed  to  find  their  own  bed.  Where  piles  are  used  and  pro- 
ject some  height  above  the  bottom,  they,  in  some  cases,  use,  be- 
sides the  loose  stone,  a  grating  of  heavy  timber,  whien  lies  between 
and  encloses  the  piling,  to  give  it  greater  stiffness  and  prevent 
outward  spreading.  In  streams  of  a  torrent  character,  where  the 
bed  is  liable  to  be  worn  away,  or  shifted,  an  artificial  covering, 
or  apron  of  stone  laid  in  mortar,  has,  in  some  cases,  been  used, 
both  under  the  arches  and  above  and  below  the  bridge,  as  far  as 
the  bed  seemed  to  require  this  protection.  At  the  bridge  of  Bor- 
deaux loose  stone  was  spread  over  the  river-bed  between  the 
piers,  and  it  has  been  found  to  answer  perfectly  the  object  of  the 
sngineer,  the  blocks  having,  in  a  few  years,  become  united  into  a 
firm  mass  by  the  clayey  sediment  of  the  river  deposited  in  their 
interstices.  At  the  elegant  cast-iron  bridge,  built  over  the  Lary 
near  Plymouth,  resort  was  had  to  a  similar  plan  for  securing  the 
bed,  which  is  of  shifting  sand.  The  engineer,  Mr.  Rendel,  here 
laid,  in  the  first  place,  a  bed  of  compact  clay  upon  the  sand  bed 
between  the  piers,  and  imbedded  in  it  loose  stone.  This  method, 
which  for  its  economy  is  worthy  of  note,  has  fully  answered  the 
expectations  of  the  engineer. 

The  English  engineers  have  greatly  improved  the  method  of 
centrng,  and,  in  their  boldest  arches,  any  settling  approaching 
that  which  the  French  engineers  usually  counted  upon,  on  striking 
their  centres,  would  now  be  regarded  as  an  evidence  of  great  de 


230  BRIDGES,  ETC. 

feet  in  the  design,  or  of  very  unskilful  workmanship.  They 
have  generally,  in  their  recent  bridges,  supported  their  roadway 
either  upon  flat  stones,  resting  on  light  walls  built  parallel  to  the 
heads,  or  else  upon  light  cylindrical  arches  laid  upon  piers  having 
the  same  direction.  In  the  preparation  for  laying  the  beds  of  their 
foundations,  they  have  generally  preferred  the  cofTer-dam  to  any 
other  plan,  although  in  many  localities  the  most  expensive,  on 
account  of  the  greater  facility  and  security  offered  by  it  for  carry- 
ing on  the  work.  They  have  not,  until  recently,  made  as  exten- 
sive an  application  of  beton  a?  the  French  for  hydraulic  purposes, 
and,  from  having  mostly  usee  what  is  known  as  concrete  among 
their  architects,  have  met  with  some  signal  failures  in  its  employ- 
ment for  these  purposes. 

WOODEN  BRIDGES. 

583.  A  wooden  bridge  consists  of  three  essential  parts:  1st, 
the  abutments  and  piers  which  form  the  points  of  support  for 
the  bridge  frame ;  2d,  the  bridge  frame  which  supports  the  su- 
perstructure between  the  piers  and  abutment ;  3d,  the  super- 
structure, consisting  of  the  roadway,  parapets,  roofing,  &c. 

584.  The  abutments  and  piers  may  be  either  of  stone,  or  of 
timber.  Stone  supports  are  preferable  to  those  of  timber,  both 
on  account  of  the  superior  durability  of  stone,  and  of  its  offering 
more  security  than  frames  of  timber  against  the  accidents  to 
which  the  piers  of  bridges  are  liable  from  freshets,  ice,  &c. 

585.  The  forms,  dimensions,  and  construction  of  stone  abut- 
ments and  piers  for  wooden  bridges  will  depend,  like  those  for 
stone  bridges,  upon  local  circumstances,  and  the  kind  of  bridge- 
frame  adopted.  If  the  bridge-frame  is  so  arranged  that  no  lateral 
thrust  is  received  from  it  by  the  piers,  the  dimensions  of  the  latter 
should  be  regulated  to  support  the  weight  of  the  bridge-frame 
and  its  superstructure,  and  to  resist  any  action  arising  from  acci- 
dental causes,  as  freshets,  ice,  &c.  The  forms  and  dimen- 
sions of  the  abutments,  under  the  like  circumstances,  will  be 
mainly  regulated  by  the  pressure  upon  them  from  the  embank- 
ments of  the  approaches. 

586.  If  the  bridge-frame  is  of  a  form  that  exerts  a  lateral 
pressure,  the  dimensions  of  the  abutments  and  piers  must  be  suit 
ably  adapted  to  resist  this  action,  and  secure  the  supports  from 
being  overturned.  Abutment-piers  may  be  used  with  advantage 
in  this  case,  as  offering  more  security  to  the  structure  than  sim- 
ple piers,  when  a  frame  between  any  two  supports  may  require 
io  be  taken  out  for  repairs.  The  starlings  should  in  all  cases  be 
carried  above  the  line  of  the  highest  water-level,  and  the  portior 
of  the  pier  above  this  line,  which  supports  the  roadway  bearers 
may  be  built  with  plane  faces  and  ends. 


WOODEN  BRIDGES. 


231 


587.  Wooden  abutments  may  be  formed  by  constructing  what 
is  termed  a  crib-work,  which  consists  of  large  pieces  of  square, 
timber  laid  horizontally  upon  each  other,  to  form  the  upright,  or 
sloping  faces  of  the  abutment.  These  pieces  are  halved  into  each 
other  at  the  angles,  and  are  otherwise  firmly  connected  togethei 
by  diagonal  ties  and  iron  bolts.  The  space  enclosed  by  the  crib- 
work,  which  is  usually  built  up  in  the  manner  just  described,  only 
on  three  sides,  is  filled  with  earth  carefully  rammed,  or  with  dry 
stone,  as  circumstances  may  seem  to  require. 

A  wooden  abutment  of  a  more  economical  construction  may 
be  made,  by  partly  imbedding  large  beams  of  timber  placed  in  a 
vertical  or  an  inclined  position,  at  intervals  of  a  few  feet  from 
each  other,  and  forming  a  facing  of  thick  plank  to  sustain  the 
earth  behind  the  abutment.  Wooden  piers  may  also  be  made 
according  to  either  of  the  methods  here  laid  down,  and  be  filled 
with  loose  stone,  to  give  them  sufficient  stability  to  resist  the 
forces  to  which  they  may  be  exposed  ;  but  the  method  is  clumsy, 
and  inferior,  under  every  point  of  view,  to  stone  piers,  or  to  the 
methods  which  are  about  to  be  explained. 

588.  The  simplest  arrangement  of  a  wooden  pier  consists 
(Fig.  127)  in  driving  heavy  square  or  round  piles  in  a  single 
-ow,  placing  them  from  two  to  four  feet  apart.     These  upright 

B 


Fig.  127— Elevation  of  a  wooden  pier. 

a,  a,  piles  of  substructure. 

b,  b,  capping  of  piles  arranged  to  receive  the  ends  of  the  uprights  c,e, 
which  support  the  string  pieces  t,  t. 

d,  upper  fender  beam. 

e,  lower  fender  beam. 

/,  horizontal  ties  bolted  in  pairs  on  the  uprights. 

f,  g,  diagonal  braces  bolted  in  pairs  on  the  uprights. 
,  capping  of  the  uprights  placed  under  the  string  pieces. 

A,  roadway 

B,  parapet. 


232 


BRIDGES,  ETC. 


pieces  are  sawed  off  level,  and  connected  at  top  by  a  horizontai 
Deam,  termed  a  cap,  which  is  either  mortised  to  receive  a  tenon 
made  in  each  upright,  or  else  is  fastened  to  the  uprights  by  bolts 
or  pins.  Other  pieces,  which  are  notched  and  bolted  in  pairs  on 
the  sides  of  the  uprights,  are  placed  in  an  inclined,  or  diagonal 
position,  to  brace  the  whole  system  firmly.  The  several  uprights 
of  the  pier  are  placed  in  the  direction  of  the  thread  of  the  current. 
If  thought  necessary,  two  horizontal  beams,  arranged  like  the 
diagonal  pieces,  may  be  added  to  the  system  just  below  the  lowest 
water-level.  In  a  pier  of  this  kind,  the  place  of  the  starlings  is 
supplied  by  two  inclined  beams  on  the  same  line  with  the  up- 
rights, which  are  termed  fender-beams. 

589.  A  strong  objection  to  the  system  just  described,  arises 
from  the  difficulty  of  replacing  the  uprights  when  in  a  state 
of  decay.  To  remedy  this  defect,  it  has  been  proposed  to  drive 
large  piles  in  the  positions  to  be  occupied  by  the  uprights,  (Fig. 
128,)  to  connect  these  piles  below  the  low- water  level  by  four 


Fig.  128— Plan  O,  elevation  M,  and  cross  section 
N,  showing  the  arrangement  of  the  capping 
of  the  foundation  piles  with  the  uprights. 

a,  piles. 

b,  capping  of  four  beams  bolted  together. 

c,  uprights. 


horizontal  beams,  firmly  fastened  to  the  heads  of  the  piles, 
which  are  sawed  off  at  a  proper  height  to  receive  the  horizontal 
beams.  The  two  top  beams  have  large  square  mortises  to  re- 
ceive the  ends  of  the  uprights,  which  rest  on  those  of  the  piles. 
The  rest  of  the  system  may  be  constructed  as  in  the  former  case. 
By  this  arrangement  the  uprights,  when  decayed,  can  be  readily 
replaced,  and  they  rest  on  a  solid  substructure  not  subject  to  de- 
cay ;  shorter  timber  also  can  be  used  for  the  piers  than  when  the 
uprights  are  driven  into  the  bed  of  the  stream. 

590.  In  deep  water,  and  especially  in  a  rapid  current,  a  single 
row  of  piles  might  prove  insufficient  to  give  stability  to  the  up 
rights ;  and  it  has  therefore  been  proposed  to  give  a  sufficient 
spread  to  the  substructure  to  admit  of  bracing  the  uprights  by  struts 
on  the  two  sides.  To  effect  this,  three  piles  (Fig.  129)  should 
be  driven  for  each  upright;  one  just  under  its  position,  and  the 
other  two  on  each  side  of  this,  on  a  line  perpendicular  to  that  of 
the  pier.  The  distance  between  the  three  piles  will  depend  on 
he  inclination  and    ength  that  it  may  be  deemed  necessary  to 


WOODEN  BRIDGES. 


239 


give  the  struts.    The  heads  of  the  three  piles  are  sawed  off  leveJ 
and  com  ected  by  two  horizontal  clamping  pieces  below  the  low 


N 


&M 


U    U    bi 


Fig.  129— Elevation  of  the  arrangement  of  a  wid« 
foundation  for  a  wooden  pier. 

a,  upright. 

b,  b,  piles  of  the  foundation. 

c,  c,  capping  of  the  piles. 

d,  a,  struts  to  strengthen  the  uprights. 

e,  e,  clamping  pieces  bolted  in  pairs  on  the  up> 
rights. 


est  water.  A  square  mortise  is  left  in  these  two  pieces,  over  the 
middle  pile,  to  receive  the  uprights.  The  uprights  are  fastened 
together  at  the  bottom  by  two  clamping  pieces,  which  rest  on 
those  that  clamp  the  heads  of  the  piles,  and  are  rendered  firmer 
by  the  two  struts. 

591.  In  localities  where  piles  cannot  be  driven,  the  uprights 
of  the  piers  may  be  secured  to  the  bottom  by  means  of  a  grating, 
arranged  in  a  suitable  manner  to  receive  the  ends  of  the  uprights. 
The  bed,  on  which  the  grating  is  to  rest,  having  been  suitably 
prepared,  it  is  floated  to  its  position,  and  sunk  either  before  or 
after  the  uprights  are  fastened  to  it,  as  may  be  found  most  con- 
venient. The  grating  is  retained  in  its  place  by  loose  stone. 
As  a  farther  security  for  the  piers,  the  uprights  may  be  covered 
by  a  sheathing  of  boards,  and  the  spaces  between  the  sheathing 
be  filled  in  with  gravel.  Wooden  piers  may  also  be  constructed, 
if  necessary,  of  two  parallel  rows  of  uprights  placed  a  few  feet 
apart,  and  connected  by  cross  and  diagonal  ties  and  braces. 

592.  As  wooden  piers  are  not  of  a  suitable  form  to  resist  heavy 
shocks,  ice-breakers  should  be  placed  in  the  stream,  opposite  to 
each  pier,  and  at  some  distance  from  it.    In  streams  with  a  gen- 

d 


Fig.  130— Elevation  M  and  plan  N  of  a  simple  I 
breaker. 

a,  a,  foundation  piles. 

b,  b,  capping  of  piles, 
e.  c,  uprights. 

a,  inclined  fender-beam  shod  with  iron. 


tie  current,  a  simple  inclined  beam  (Fig.  1 30)  covered  with  thick 
ulieet  iron,  and  supported  by  uprights  and  diagonal  pieces,  will 

30 


234 


BRIDGES,  ETC. 


be  all  that  is  necessary  for  an  ice-breaker.    But  in  rapid  currents 
a  crifr-work,  having  the  form  of  a  triangular  pyramid,  (Fig.  131, 


Fig.  131— Elevation  M  and  plan  N  of  th« 
frame  of  an  ice-breaker  to  be  filled  ir« 
with  broken  stone 


the  up-stream  edge  of  which  is  covered  with  iron,  will  be  re- 
quired, to  offer  sufficient  resistance  to  shocks.  '  The  crib-work 
may  be  filled  in,  if  it  be  deemed  advisable,  with  blocks  of  stone. 

593.  The  width  of  the  bays  in  wooden  bridges  will  depend  on 
the  local  circumstances.  As  a  general  rule,  the  bays  may  be 
wider,  and  in  bridge-frames  of  curved  timber  the  rise  less,  than 
in  stone  bridges.  In  arranging  this  point,  the  engineer  must  take 
into  consideration  the  fact  that  wooden  bridges  require  more  fre- 
quent repairs  than  those  of  stone,  arising  from  the  decay  of  the 
material,  and  from  the  effects  of  shrinking  and  vibrations  upon 
the  joints  of  the  frames,  and  that  the  difficulty  of  replacing  de- 
cayed parts,  and  readjusting  the  frame-work,  increases  rapidly 
with  the  span. 

594.  Bridge-frames  may  be  divided  into  two  general  classes. 
To  the  one  belong  all  those  combinations,  whether  of  straight  or 
of  curved  timber,  that  exert  a  lateral  pressure  upon  the  abutments 
and  piers,  and  in  which  the  superstructure  is  generally  above  the 
bridge-frame.  To  the  other,  those  combinations  which  exert  no 
lateral  pressure  upon  the  points  of  support,  and  in  which  the  road 
way,  &c.  may  be  said  to  be  suspended  from  the  bridge-frame. 

595.  Any  of  the  combinations,  whether  of  straight,  or  of  curved 
timber,  described  under  the  head  of  Framing,  may  be  used  for 
bridges,  according  to  the  width  of  bay  selected.  A  preterence, 
within  late  years,  has  been  generally  given  by  engineers  to  com- 
binations of  straight  timber  over  curved  frames,  from  the  greater 
simplicity  and  facility  of  their  construction,  as  well  as  their 
greater  economy ;  as  curved  frames  require  much  moie  iron  in 


WOODEN  BRIDGES.  235 

ihe  form  of  bolts,  ties,  &c,  than  frames  of  straight  timber,  and 
more  costly  mechanical  contrivances  for  putting  the  parts  together, 
and  setting  the  frame  upon  its  supports. 

590.  The  number  of  ribs  in  the  bridge-frame  will  depend  on 
the  general  strength  required  by  the  object  of  the  structure,  and 
upon  the  class  of  frame  adopted.  In  the  first  class,  in  which  the 
roadway  is  usually  above  the  frames,  any  requisite  number  of  ribs 
may  be  used,  and  they  may  be  placed  at  equal  intervals  apart, 
or  else  be  so  placed  as  to  give  the  best  support  to  the  loads  which 
pass  over  the  bridge.  In  the  second  class,  as  the  frame  usually 
lies  entirely,  or  projects  partly  above  the  roadway,  &c,  if  more 
than  two  ribs  are  required,  they  are  so  arranged  that  one  or  two, 
as  circumstances  may  demand,  form  each  head  of  the  bridge,  and 
one  or  two  more  are  placed  midway  between  the  heads,  so  as  to 
leave  a  sufficient  width  of  roadway  between  the  centre  and  adja- 
cent ribs.  The  footpaths  are  usually,  in  this  case,  either  placed 
between  the  two  centre  ribs,  or,  when  there  are  two  exterior  ribs, 
between  them. 

597.  The  manner  of  constructing  the  ribs,  and  of  connecting 
them  by  cross  ties  and  diagonal  braces,  is  the  same  for  bridge 
frames  as  for  other  wooden  structures  ;  care  being  taken  to  ob- 
tain the  strength  and  stiffness  which  are  peculiarly  requisite  in 
wooden  bridges,  to  preserve  them  from  the  causes  of  destructi 
bility  to  which  they  are  liable.  In  frames  which  exert  a  lateral 
pressure  against  the  abutments  and  piers,  the  lowest  points  of 
the  frame-work  should  be  so  placed  as  to  be  above  the  ordinary 
high-water  level ;  and  plates  of  some  metal  should  be  inserted  at 
those  points,  both  of  the  frame  and  of  the  supports,  where  the 
effect  of  the  pressure  might  cause  injury  to  the  woody  fibre. 

59S.  The  roadway  usually  consists  of  a  simple  flooring  formed 
of  cross  joists,  termed  the  roadway-bearers,  or  floor-girders,  and 
flooring-boards,  upon  which  a  road-covering  either  of  wood,  or 
stone  is  laid.  A  more  common  and  better  arrangement  of  the 
roadway,  now  in  use,  consists  in  laying  longitudinal  joists  of 
smaller  scantling  upon  the  roadway-bearers,  to  support  the 
flooring-boards.  This  method  preserves  more  effectually  than 
the  other  the  roadway-bearers  from  moisture.  Besides,  in 
bridges  which,  from  the  position  of  the  roadway,  do  not  admit 
of  vertical  diagonal  braces  to  stiffen  the  frame-work,  the  only 
means,  in  most  cases,  of  effecting  this  object  is  in  placing  hori- 
zontal diagonal  braces  between  each  pair  of  roadway-bearers 
For  like  reasons,  ston°.  road-coverings  for  wooden  bridges  are 
generally  rejected,  and  one  of  plank  used,  which,  for  a  horse 
track,  should  be  of  two  thicknesses,  so  that,  in  case  of  repairs, 
arising  from  the  wear  and  tear  of  travel,  the  boards  resting  upon 
the  flooring-joists  may  not  require  to  be  removed     The  footpaths 


236  BRIDGES,  ETC. 

consist  simply  of  a  slight  flooring  of  sufficient  width,  which  ?• 
usually  detached  from  and  raised  a  few  inches  above  the  roadwa> 
surface. 

599.  When  the  bridge-frame  is  beneath  the  rcadway,  a  distinct 
parapet  will  be  requisite  for  the  safety  of  passengers.  This  may 
be  formed  either  of  wood,  of  iron,  or  of  the  two  combined.  It  is 
most  generally  made  of  timber,  and  consists  of  a  hand  and  foot 
rail  connected  by  upright  posts  and  stiffened  by  diagonal  braces 
A  wooden  parapet,  besides  the  security  it  gives  to  passengers, 
may  be  made  to  add  both  to  the  strength  and  stiffness  of  the 
bridge,  by  constructing  it  of  timber  of  a  suitable  size,  and  con- 
necting it  firmly  with  the  exterior  ribs. 

600.  In  bridge  frames  in  which  the  ribs  are  above  the  roadway, 
a  timber  sheathing  of  thin  boards  will  be  requisite  on  the  sides, 
and  a  roof  above,  to  protect  the  structure  from  the  weather.  The 
tie-beams  of  the  roof-trusses  may  serve  also  as  ties  for  the  ribs 
at  top,  and  may  receive  horizontal  diagonal  braces  to  stiffen  the 
structure,  like  those  of  the  roadway-bearers.  The  rafters,  in  the 
case  in  which  there  is  no  centre  rib,  and  the  bearing,  or  distance 
between  the  exterior  ribs,  is  so  great  that  the  roadway-bearers  re- 
quire to  be  supported  in  the  middle,  may  serve  as  points  of  sup- 
port for  suspension  pieces  of  wood,  or  of  iron,  to  which  the  middle 
point  of  the  roadway-bearers  may  be  attached. 

601.  When  the  bridge-frame  is  beneath  the  roadway,  the  floor- 
ing, if  sufficient  projection  be  given  it  beyond  the  head,  will  pro- 
tect it  from  the  weather,  if  the  depth  of  the  ribs  be  not  very  great. 
In  the  contrary  case  a  side  sheathing  of  boards  may  be  requisite. 

602.  The  frame  and  other  main  timbers  of  a  wooden  bridge 
will  not  require  to  be  coated  with  paint,  or  any  like  composition, 
to  preserve  them  from  decay  when  they  are  roofed  and  boarded 
in  to  keep  them  dry.  When  this  is  not  the  case,  the  ordinary 
preservatives  against  atmospheric  action  may  be  used  for  them. 
The  under  surface  and  joints  of  the  planks  of  the  roadway  may 
be  coated  with  bituminous  mastic  when  used  for  a  horse-track ; 
in  railroad  bridges  a  metallic  covering  may  be  suitably  used 
when  the  bridge  is  not  traversed  by  horses. 

603.  Wooden  bridges  can  produce  but  little  other  architectural 
effect  than  that  which  naturally  springs  up  in  the  mind  of  an 
educated  spectator  in  regarding  any  judiciously-contrived  struc- 
ture. When  the  roadway  and  parapet  are  above  the  bridge- 
frame,  a  very  simple  cornice  may  be  formed  by  a  proper  combi- 
nation of  the  roadway-timbers  and  flooring,  which,  with  the  para- 
pet, will  present  not  only  a  pleasing  pppearance  to  the  eye,  but 
will  be  of  obvious  utility  in  covering  trie  parts  beneath  from  the 
weather.  In  covered  bridges,  the  most  that  can  be  done  will  be 
to  paint  them  with  a  uniform  coat  of  some  subdued  tint.     A 


WOODEN  BRIDGES. 


237 


best,  from  their  want  of  height  as  compared  with  their  length, 
covered  wooden  bridges  must,  for  the  must  part,  be  only  unsightly 
and  also  apparently  insecure  structures  when  looked  at  from  such 
a  point  of  view  as  to  embrace  all  the  parts  in  the  field  of  vision  ; 
and  any  attempt,  therefore,  to  disguise  their  true  character,  and  to 
give  them  by  painting  the  appearance  of  houses,  or  of  stone  arches, 
while  it  must  fail  to  deceive  even  the  most  ignorant,  will  only  be- 
tray the  bad  taste  of  the  architect  to  the  more  enlightened  judge. 

604.  The  art  of  erecting  wooden  bridges  has  been  carried  to 
great  perfection  in  almost  every  part  of  the  world  where  timber 
has,  at  any  period,  been  the  principal  building  material  at  the 
disposal  of  the  architect.  The  more  modern  wooden  bridges  of 
Switzerland  and  Germany  occupy  in  Europe  the  first  rank,  for 
boldness  of  design  and  scientific  combination  in  their  arrangement 
and  construction.  These  fine  foreign  structures  have  been  even 
surpassed  in  the  United  States,  and  our  wooden  bridges  and  the 
skill  of  our  engineers  and  carpenters,  as  shown  through  them, 
have  become  deservedly  celebrated  tliroughout  the  scientific 
world.  The  more  recent  structures  of  this  class  are  peculiarly 
characterized  for  simplicity  of  arrangement,  perfection  in  the  me- 
chanical execution,  and  boldness  of  design.  If  they  are  open  to 
the  charge  of  any  fault,  it  is  to  that  of  too  great  boldness  of  de- 
sign, in  spanning  very  wide  bays  with  ribs  of  open-built  beams 
either  unsupported,  or  but  imperfectly  so,  at  intermediate  points, 
by  any  combination  of  struts  and  corbels,  or  straining  beams. 
The  want  of  these  additions  is  more  or  less  apparent  in  the  great 
vibratory  motion  felt  on  some  of  the  more  recent  railroad  and 
other  bridges,  and  in  a  consequent  disposition  in  the  frame  to 
work  loose  at  the  joints  and  sag. 

005.  The  following  Table  contains  the  principal  dimensions 
of  some  of  the  most  celebrated  American  and  European  wooden 
bridges. 


NAME,    KTC.   OF   BBIDOK. 

Number  of 
bays. 

Width  of 
widest  bay. 

Rise  or  depth 
of  rib. 

Bridge  of  SchafFhausen,  (A) 
!  Bridge  of  Kandel,  (B)    .       . 
Bridge  of  Bamberg,       >  /p. 
Bridge  of  Freysingen,  J 
Essex  bridge,  (D)     . 
Upper  Schuylkill  bridge,  (E) 
Market-street  bridge,  (F) 
Trenton  bridge,  (G) 
i  Columbia  bridge,  (H) 
Richmond  bridge,  (I) 
Springfield  bridge,  (K)   . 

2 
1 
1 
2 
1 
1 
3 
5 
29 
19 
7 

193  ft. 
166  " 
208  « 
153  " 
250  " 
340  " 
195    ' 
200  " 
200  " 
153  " 
180  - 

16.9  ft. 
11.6  " 

20      " 
12      " 
27      " 

15.4  ** 
18      ■ 

238  BRIDGES,  ETC. 

(A)  This  celebrated  Swiss  bridge,  built  by  John  Ulrich  Gra 
benmann,  a  carpenter,  consisted  of  two  bays,  the  one  193  and 
the  other  172  feet.  The  bridge-frame  was  formed  of  two  ribs 
with  a  roadway  between  them.  Each  rib  was  framed,  in  some 
respects,  on  trie  same  principle  as  an  open-built  beam,  the  upper 
string  being  supported  by  a  number  of  inclined  struts  which 
rested  against  the  abutments  and  pier,  and  the  lower  string,  upon 
which  the  roadway  timbers  were  laid,  being  suspended  from  the 
upper  by  suspension  pieces.  The  whole  structure  was  consoli- 
dated and  braced  by  bolts,  stays,  and  straps  of  iron.  Remarkable 
in  its  day,  yet  the  drawings  extant  of  the  bridge  of  Schaff  hausen, 
while  they  attest  the  ingenuity  and  practical  skill  of  the  builder, 
present  it  in  singular  contrast  with  the  equally  bold  and  less  com- 
plicated structures  of  the  like  nature  recently  erected  in  the 
United  States. 

(B)  This  is  also  a  Swiss  bridge,  built  over  the  torrent  of  Kan- 
del  in  the  canton  of  Berne.  Its  ribs  are  formed  of  solid-built 
beams  which  gradually  decrease  in  depth  from  the  centre  to  the 
extremities  ;  this  decrease  being  made  by  offsets,  the  built  beams 
presenting  the  appearance  of  a  number  of  straining  beams  placed 
below  each  other,  against  the  ends  of  which  abut  inclined  struts 
that  rest  against  the  faces  of  the  abutments.  The  roadway  rests 
upon  the  built  beams. 

(C)  These  two  bridges  are  selected  from  among  a  number  of 
the  like  character  constructed  in  various  parts  of  Germany  by 
Wiebeking.  The  bridge-frame  in  all  of  them  consists  of  several 
ribs  of  curved  solid-built  beams  upon  which  the  roadway  timbers 
are  laid.  This  method  of  constructing  bridge-frames  combines 
great  strength  and  stiffness.  It  is  more  expensive  than  frames  of 
straight  timber,  as  it  requires  a  larger  amount  of  iron,  and  more 
complicated  mechanical  means  for  its  construction  than  the  latter, 
and  the  ribs,  although  stiffer,  are  impaired  in  strength  by  the 
operation  of  bending  them. 

(D)  This  is  a  very  remarkable  structure  built  over  the  river 
Merrimack  near  Newburyport.  The  ribs  consist  of  curved  open- 
built  beams,  each  of  which  is  composed  of  three  concentric  solid- 
built  beams,  connected,  at  intervals  along  the  rib,  by  two  radial 
pieces  of  hard  wood  which  fit  into  mortises  made  through  the 
centre  of  each  solid  beam,  and  by  a  long  wedge  of  hard  wood  in- 
serted, in  the  direction  of  the  radius  of  curvature,  between  each 
pair  of  radial  pieces.  Each  of  the  solid-built  beams  of  the  rib  is 
formed  of  two  thicknesses  of  scantling,  about  12  or  15  feet  in 
length,  which  abut  end  to  end,  breaking  joints,  and  are  connected 
by  keys  of  hard  wood  inserted  into  mortises  made  through  the  two 
thicknesses.  By  these  arrangements  the  architect  has  sought 
to  preserve  both  the  curved  shape  and  the  parallelism  of  the  solid 


WOODEN  BRIDGES. 

beams  forming  the  rib,  and  also  to  connect  all  the  parts  firmly, 
The  combination  is  an  ingenious  attempt  at  constructing  an  arch 
of  wood  on  similar  principles  to  one  of  stone,  but  is  inferior  tc 
the  more  simple  and  usual  methods  of  forming  curved  open-built 
beams  by  using  radial  and  diagonal  pieces. 

(E)  This  bridge,  designed  and  built  by  L.  Wernwag,  has  the 
widest  span  on  record.     The  bridge-frame  (Fig.  132)  consists 


Fig.  132— Represents  a  side  view  of  a  portion  of  the  open  curved  rib  of  the 
bridge  over  the  Schuylkill  at  Philadelphia. 

A,  lower  curved  solid  beam. 

B,  top  beam. 
a,  a,  posts. 

c,  c,  diagonal  braces. 

o,  o,  iron  diagonal  ties. 

7n,  m,  iron  stays  anchored  in  the  abutment  C. 

of  five  ribs.  Each  rib  is  an  open-built  beam  formed  of  a  bottom, 
curved  solid-built  beam  and  of  a  single  top  beam,  which  are  con- 
nected by  radial  pieces,  diagonal  braces,  and  inclined  iron  stays. 
The  bottom  curved  beam  is  composed  of  three  concentric  solid- 
built  beams  slightly  separated  from  each  other,  each  of  which 
has  seven  courses  of  curved  scantling  in  it,  each  course  6  inches 
thick  by  13  inches  in  breadth ;  the  courses,  as  well  as  the  con- 
centiic  beams,  being  firmly  united  by  iron  bolts,  &c.  A  road 
■way  that  rests  upon  the  bottom  curved  ribs  is  left  on  each  side 
if  the  centre  rib,  and  a  footpath  between  each  of  the  two  exterior 
ribs.  The  bridge  was  covered  in  by  a  roof  and  a  sheathing  on 
the  sides. 

(F)  This  is  also  one  of  the  many  bridges  designed  and  built 
by  Wernwag  in  the  States  of  Pennsylvania  and  New  Jersey. 
The  bridge-frame  consists  of  three  ribs  placed  so  far  apart  as  to 
leave  space  between  them  for  a  carriage-way  and  a  footpath  on 
each  side  of  the  centre  rib.  Each  rib  is  an  open-built  beam, 
consisting  of  a  bottom  curved  solid-built  beam,  with  mortises  at 
intervals  to  receive  radial  pieces,  which  are  connected  at  top  by 
a  single  beam,  also  mortised  to  receive  tenons  on  the  heads  of  the 
radial  pieces.     A  single  diagonal  brace  »  placed  between  each 


240 


BRIDGES.  ETC. 


pair  of  radial  pieces.  Longitudinal  beams  reach  from  the  crown 
of  the  curved  rib  of  one  bay  to  that  of  the  next,  and  on  these  the 
roadway-bearers  are  laid  transversely. 

(G)  In  this  fine  structure,  the  roadway-bearers  are  suspended 
from  curved  solid-built  beams  by  iron-bar  chains  and  suspension 
rods.  The  span  of  the  centre  bay  is  200  feet,  that  of  the  twc 
adjacent  180  feet,  and  that  of  the  extreme  bays  160  feet.  The 
bridge-frame  (Fig.  133)  consists  of  five  ribs,  having  the  same 


Fig.  133— Represents  a  side  view  of  a  portion  of  a  rib  of  Trenton  bridge. 

Ai  solid  curved  beam. 

a,  a,  a,  cross  girders  suspended  from  A  by  the  iron  chains  b,  b. 

c,  c,  roadway-bearers. 

d,  a,  diagonal  braces. 

B.  portion  of  pier. 

C,  frame  work  of  roof-covering  over  the  piers. 

arrangement  for  the  roadway  and  footpaths  as  in  the  upper  Schuyl- 
kill bridge.  Each  of  the  central  ribs  is  formed  of  8  courses  of 
curved  scantling,  each  course  being  4  inches  thick,  and  1 3  inches 
broad.  The  two  exterior  ribs  have  9  courses  of  scantling  of  the 
same  dimensions.  Inclined  timber  braces  connect  the  curved 
beam. and  roadway  timbers.  The  ribs  are  tied  at  top  by  cross 
pieces,  and  stiffened  by  diagonal  braces.  The  bridge-frame  is 
braced  on  the  exterior  by  vertical  and  horizontal  timber-stays 
which  abut  against  the  top  of  the  piers.  The  roadway  is  of 
plank  laid  upon  longitudinal  joists  that  rest  on  the  roadway 
bearers.  The  roadway-bearers  are  stiffened  by  diagpnal  braces. 
The  abutments  and  piers  are  of  stone,  the  latter  being  20  feet 
thick  at  the  impost. 

(H)  This,  like  most  of  the  more  recent  bridges  for  railroads, 
aqueducts,  &c,  in  Pennsylvania,  is  built  upon  Burr's  plan,  which 
{Fig.  134)  consists  in  forming  each  rib  of  an  open-built  beam  of 
straight  timber,  and  connecting  with  it  a  curved  solid-built  beam 
formed  of  two  or  more  thicknesses  of  scantling,  between  which 
the  frame-work  of  the  open-built  beam  is  clamped.  The  open- 
built  beam  consists  of  a  horizontal  bottom  beam  of  two  thicknesses 
of  scantling,  termed  the  chords,  which  clamp  uprights,  termed  the 


WOODEN   BRIDOES 


241 


queen  posts,  between  them, — of  a  single  top  beam,  termed  the  plat: 
of  the  side  frame,  which  rests  upon  the  uprights,  with  which  it  i ■ 


Fig.  134— Represents  a  side  view  of  a 

portion  of  a  rib  of  Burr's  bridge. 
a,  a,  arch  timbers. 
d,  d,  queen-posts. 
b,b,  braces. 
c,  c,  chords. 


e,  e,  plate  of  the  side  frame. 

o,  o,  floor  girders  on  which  the  flooring 

joists  and  flooring  boards  rest. 
n,  n,  check  braces, 
t,  i,  tie  beams  of  roof. 
A,  portion  of  pier. 


connected  by  a  mortise  and  tenon  joint, — and  of  diagonal  braces 
and  other  smaller  braces,  termed  check  braces,  placed  between 
the  uprights.  The  curved-built  beam,  termed  the  arch-timbers, 
is  bolted  upon  the  timbers  of  the  open-built  beam.  The  bridge- 
frame  may  consist  of  two  or  more  ribs,  which  are  connected  and 
stiffened  by  cross  ties  and  diagonal  braces.  The  roadway-floor- 
ing (Fig.  135)  is  laid  upon  cross  pieces,  termed  the  floor  girders, 
which  may  either  rest  upon  the  chords,  or  else  be  attached  at  any 
intermediate  point  between  them  and  the  top  beam.  The  road 
way  and  footpaths  may  be  placed  in  any  position  between  the 
several  ribs. 

There  is  great  similarity  between  the  combination  adopted  by 
Burr  and  those  of  the  two  bridge-frames  just  described.  The 
main  difference  consists  in  the  application  by  Burr  of  wL-it  he 
terms  the  arch-timbers,  to  strengthen  and  stiffen  an  open-bum 
beam.  It  may  be  remarked  from  the  Figs.  134,  135,  that,  the 
framing  of  the  open-built  beam  is  faulty,  in  that  the  top  beam,  or 
plate,  is  not  only  of  less  dimensions  than  the  bottom  beam,  or 
chord,  but  is  weakened  by  mortises,  and  moreover  affords  no 
other  support  to  the  queen  posts,  or  uprights,  which  act  as  sus- 
pension pieces  for  the  chord,  than  that  of  ihe  pip.  which  confines 
the  tenon  in  the  mortise.  From  the  manner  in  which  the  arsh- 
timbers  are  formed  and  connected  with  the  parts  of  the  open-tuilt 

31 


242 


BRIDGES,  ETC. 


,beam,  they  add  but  little  if  any  more  strength  and  stiffness  tha 
would  be  given  by  straight  timbers  reaching  from  the  springing 
point  of  the  arch  timbers  to  their  crown ;  and  they  are  certainly 


Fig.  135— Represents  the  half  of  a 
cross  section  of  Fig.  134  through 
the  crown  of  the  arch  timbers,  in 
which  the  letters  designate  the 
same  parts  as  in  the  preceding  Fig. 

f,  rafters  of  roof  truss. 
,  A,  diagonal  braces  of  bridge  frame. 
B,  side  view  of  the  pier. 


less  efficacious  in  subserving  their  end  than  would  be  inclined 
struts,  occupying  a  like  position  at  bottom,  and  abutting  against  a 
straining  beam,  placed  either  under  the  centre  part  of  the  chord, 
where  the  locality  would  permit  it,  or  under  the  centre  portion  of 
the  plate.  In  localities  where  fine  timber  is  less  abundant  than  in 
those  in  which  the  most  of  Burr's  bridges  have  been  built,  a  ju- 
dicious regard  to  economy  would  undoubtedly  have  suggested  a 
selection  of  forms  for  the  secondary  parts  of  the  frame,  which 
would  have  prevented  these  parts  from  being  as  much  cut  to 
waste  as  the  Figs,  show  they  must  have  been  in  the  example 
taken  to  illustrate  this  system. 

(I)  This  structure,  constructed  under  the  superintendence  of 
Moncure  Robinson,  Esq.,  is  upon  Town's  plan.  The  width  of 
the  bays  varies  from  130  to  153  feet.  It  consists  of  two  ribs, 
each  of  which  is  formed  of  a  double  lattice,  with  two  chords  at 
bottom  and  one  at  top.  The  roadway,  for  rails,  rests  on  the  top 
girders.  The  ribs  are  braced  by  vertical  diagonal  braces,  and  by 
horizontal  diagonal  braces  between  each  pair  of  the  top  and  bot- 
tom girders.  The  />iers  are  of  rustic  work ;  they  are  40  feet 
above  the  low-water  level,  and  4  feet  thick  at  top.  The  exam- 
ple here  selected  for  illustration  (Fig.  136)  is  taken  from  another 
bridge,  of  nearly  the  same  width  of  bay,  erected  subsequently 
vo  the  Richmond  bridge,  by  the  same  engineer,  in  which  the  top 


WOODEN  BRIDGES. 


K43 


chord  also  is  doubled,  as  the  former  bridge  was  found  to  be  rcthei 
weak.  • 


Fig.  13&— Represents  a  cross  section  of  a  railroad 
Bridge  with  Town's  truss,  in  which  the  rai's  of 
the  road  are  placed  on  a  flooring  on  the  top  of 
the  truss. 

a,  a,  double  lattice. 

c,  c,  top  and  bottom  chords  doubled. 

b,  b,  vertical  diagonal  braces  of  the  two  ribs. 

d,  d,  top  and  bottom  girders. 

e,  e,  flooring  joists. 

n,  cross  bearers  of  the  rails. 

o,  o,  corbels  or  support  timbers  on  the  piers  on 

which  the  ribs  rest. 
/,  sheathing,  or  weather-boarding. 


(K ;  This  bridge  is  constructed  on  Howe's  plan.  It  consist? 
[Fig.  1  *7)  of  two  ribs  which  are  connected  at  top  and  bottom,  in 
the  usu-it  manner,  with  cross  ties  and  diagonal  braces.  The 
roadway  flooring  rests  upon  the  cross  girders  at  bottom.  The 
bridge  is  not  roofed,  as  is  usually  the  case,  the  ribs  being  covered 
in  on  the.  sides  and  at  top  by  a  sheathing  of  boards,  and  the 
flooring-boards  by  a  metallic  covering. 

The  bridges  constructed  according  to  Colonel  Long's  plan 
have  been  mostly  applied  to  medium  spans.  In  the  printed  de- 
scription of  the  different  improvements  of  this  system  patented 
by  Colonel  Long,  he  very  judiciously  introduces  struts,  which 
he  terms  arch  braces,  either  below  the  top  or  the  bottom  string, 
as  the  locality  may  demand,  for  the  purpose  of  preventing  sag- 
ging, which  must  necessarily  take  place  in  time  in  all  open-buut 
oeams  of  considerable  span,  if  not  strengthened  in  this  way. 


£44 


J1RIDGES,  ETO. 


ii    !!li;|i|!i  II II  I! i:  II  :l  II !!  II  li  i!  ii III II II 


Fig.  137— Represents  a  side  view  of  the  truss  and  an  end  view  of  the  pier,  M  ;  a  cross 
section  of  the  truss  and  side  view  of  the  pier,  N  ;  and  a  top  view  0,  of  the  pier  of 
the  railroad  bridge  at  Springfield. 

A,  inclined  plane  of  the  ice-breaker  of  the  up-stream  starling. 

a,  a,  iron  side-stays  of  the  ribs  anchored  into  the  piers  at  top. 

CAST-IRON  BRIDGES. 

606.  Bridges  of  cast  iron  admit  of  even  greater  boldness  of 
design  than  those  of  timber,  owing  to  tiie  superiority,  both  in 
strength  and  durability,  of  the  former  over  the  latter  material ; 
and  they  may  therefore  be  resorted  to  under  circumstances  very 
nearly  the  same  in  which  a  wooden  structure  would  be  suitable. 

607.  The  abutments  and  piers  of  ca6t-iron  bridges  should  be 
built  of  stone,  as  the  corrosive  action  of  salt  water,  or  even  of 
fresh  water  when  impure,  would  in  time  render  iron  supports  of 
this  character  insecure  ;  and  timber,  when  exposed  to  the  same 
destructive  agents,  is  still  les*  iurable  than  cast  iron. 

The  forms  and  dimensions  ~f  the  stone  abutments  and  piers 
are  regulated  on  the  same  principles  as  the  like  parts  in  wooden 
bridges  with  curved  frames.  The  piers  may  be  either  built  up 
high  enough  to  receive  the  roadway-bearers,  or  else  they  may  be 
terminated  just  above  the  springing  plates  of  the  bridge-frame, 


CAST-IRON  BRIDGES.  24S 

and  form  supports  for  cast-ire  in  standards  upon  which  the  roadway 
bearers  may  be  laid. 

608.  The  curved  ribs  of  cast-iron  bridge-frames  have  under 
gone  various  modifications  and  improvements.  In  the  earliei 
bridges,  they  were  formed  of  several  concentric  arcs,  or  curved 
beams,  placed  at  some  disfci.ee  asunder,  and  anited  by  radial 
pieces  ;  the  spandrels  being  filled  either  by  contiguous  rings,  or 
by  vertical  pieces  of  cast  iron  upon  which  the  roadway  bearers 
were  laid. 

In  the  next  stage  of  progress  towards  improvement,  the  curved 
ribs  were  made  less  deep,  and  were  each  formed  of  several  seg- 
ments, or  panels  cast  separately  in  one  piece,  each  panel  con- 
sisting of  three  concentric  arcs  connected  by  radial  pieces,  and 
having  flanches,  with  other  suitable  arrangements,  for  connecting 
them  firmly  by  wrought-iron  keys,  screw-bolts,  &c. ;  the  entire 
rib  thus  presenting  the  appearance  of  three  concentric  arcs  con- 
nected by  radial  pieces.  The  spandrels  were  filled  either  with 
panels  formed  like  those  of  the  curved  ribs,  with  iron  rings,  or 
with  a  lozenge-shaped  reticulated  combination.  The  ribs  were 
connected  by  cast-iron  plates  and  wrought-iron  diagonal  ties. 

In  the  more  recent  structures,  the  ribs  have  been  composed  of 
voussoir-shaped  panels,  each  formed  of  a  solid  thin  plate  with 
flanches  around  the  edges  ;  or  else  of  a  curved  tubular  rib,  formed 
like  those  of  Polonceau,  or  of  Delafield,  described  under  the  head 
of  Framing.  The  spandrel-filling  is  either  a  reticulated  combi- 
nation, or  one  of  contiguous  iron  rings.  The  ribs  are  usually 
united  by  cast-iron  tie-plates,  and  braced  by  diagonal  ties  of  cast 
and  wrought  iron. 

609.  The  roadway-bearers  and  flooring  may  be  formed  either 
of  timber,  or  of  cast  iron.  In  the  more  recent  structures  in  Eng- 
land, they  have  been  made  of  the  latter  material ;  the  roadway- 
bearers  being  cast  of  a  suitable  form  for  strength,  and  for  their 
connection  with  the  ribs ;  and  the  flooring-plates  being  of  cast 
iron. 

The  roadway  and  footpaths,  formed  in  the  usual  manner,  rest 
upon  the  flooring-plates. 

The  parapet  consists,  in  most  cases,  of  a  light  combination  of 
cast  or  wrought  iron,  in  keeping  with  the  general  style  of  the 
structure. 

610.  The  English  engineers  have  taken  the  lead  in  this  branch 
of  architecture,  and,  in  their  more  recent  structures,  have  carried 
it  to  a  high  degree  of  mechanical  perfection  and  architectura' 
elegance.  Among  the  more  celebrated  cast-iron  bridges  in  Eng 
land,  that  of  Coalbrookdale  belongs  to  the  first  epoch  above  men 
tioned ;  those  of  Staines  and  Sunderland  to  the  second ;  and  tc 
the  third,  the  bridge  of  Southward,  at.  London ;  that  of  Tewkes 


846 


BRIDGES,  ETC. 


bury  over  the  Severn  ;  that  over  the  La.y  near  Plymouth,  and  & 
number  of  others  in  various  parts  of  the  United  Kingdom. 

The  French  engineers  have  not  only  followed  the  lead  set  them 
by  the  English,  but  have  taken  a  new  step,  in  the  tubular-shaped 
ribs  of  M.  Polonceau.  The  Pont  des  Arts  at  Paris,  a  very  ligh 
bridge  for  foot-passengers  only,  and  which  is  a  combination  ol 
cast  and  wrought  iron,  belongs  to  their  earliest  essays  in  this  line  ; 
the  Pont  d'Austerlitz,  also  at  Paris,  which  is  a  combination  simi- 
lar to  those  of  Staines  and  Sunderland,  belongs  to  their  second 
epoch ;  and  the  Pont  du  Carrousel,  in  the  same  city,  built  upon 
Polonceau's  system,  with  several  others  on  the  same  plan,  belong 
to  the  last. 

In  the  United  States  a  commencement  can  hardly  be  said  to 
have  been  made  in  this  branch  of  bridge  architecture  ;  the  bridge 
of  eighty  feet  span,  with  tubular  ribs,  constructed  by  Major  Dela- 
fleld  at  Brownsville,  stands  almost  alone,  and  is  a  step  contem- 
porary with  that  of  Polonceau  in  France. 

The  following  Table  contains  a  summary  description  of  some 
of  the  most  noted  European  cast-iron  bridges. 


NAME  Or  BRIDGE. 

Hirer. 

Numb. 
of 

arcliee. 

Span  in 

lee  i. 

Rise  in 
feet. 

Numb, 
of  ribs. 

Date. 

Engineer. 

Coalhrookdale,  (A)  . 

Severn. 

1 

100.5 

40 

5 

1779 

Wearmouth,  (B)      . 

Wear. 

1 

240 

30 

6 

1796 

Bunion. 

Staines,  (C)         ... 

— 

1 

181 

16  J 

- 

1802 

— 

Austerlitz,  (D)   .      .      . 

Seine. 

5 

106.6 

10.6 

7 

1805 

Lamande. 

Vauxhall,  (E)    .      .      . 

Thames. 

9 

78 

- 

9 

1816 

Walker. 

Southwark,  (F)        .      . 

Thames. 

3 

240 

24 

8 

1818 

Rennie. 

Tewkesbury,  (G)     .      . 

Severn. 

1 

170 

17 

6 

- 

Telford. 

Lary.  (H)     .      .      .      . 

I.ary. 

5 

100 

14.5 

5 

1827 

Rendel. 

j  Carrousel,  (1)     .      .      . 

Seine. 

3 

150 

16 

5 

1838 

Polonceau. 

(A)  This  is  the  first  cast-iron  bridge  erected  in  England.  The 
curved  rib  is  nearly  a  semicircle  in  shape,  and  is  composed  of 
three  concentric  arcs,  which  are  connected  at  intervals  by  short 
columnar  pieces,  in  the  direction  of  the  radii  of  the  curve. 

(B)  This  structure,  which  connects  Wearmouth  and  Sunder- 
land, has  a  remarkably  bold  appearance,  both  from  its  great  span, 
and  its  height,  which  is  100  feet  between  the  high  water-level 
and  the  intrados  of  the  arch  at  the  crown.  The  entire  rib  pre- 
sents the  appearance  of  an  open-built  beam,  composed  of  three 
concentric  arcs  united  by  radial  pieces.  The  spandrel-filling  is 
formed  of  contiguous  iron  rings,  of  increasing  diameters  from  the 
crown  to  the  springing  line,  which  rest  upon  the  back  of  the 
curved  rib,  and  support  the  roadway-bearers. 

(C)  Staines  bridge  was  designed  on  the  same  plan  as  Wear- 
month  ;  but  from  a  defect  in  the  strength  of  its  abutments,  they 
successively  yielded  to  the  hori?  .mtal  thrust,  which  in  so  flat  an 
arch  was  very  considerable. 


CAST-IRON  BRIDGES.  24"? 

(D)  The  bridge  of  Austerlitz  is  constructed  on  the  same  prin 
:-Wif*  as  the  two.  last,  and  produces  a  light  and  pleasing  architec- 
tural effect.  Each  curved  rib  consists  of  21  voussoir-shaped 
panels,  about  4  feet  in  depth.  The  spandrel-fillings  present  the 
appearance  of  a  continuation  of  the  curved  rib  outwards,  to  form 
a  support  for  the  roadway-bearers.  The  piers  are  terminated  at 
the  springing  lines  of  the  curved  rib,  and  are  at  this  point  13  feet 
thick ;  the  roadway  above  them  being  supported  by  the  ribs  con 
tinued  up  to  its  level.  The  roadway  is  on  a  level,  the  roadway 
nearers  and  flooring  being  of  timber. 

(E)  In  this  structure  the  curved  rib  is  formed  of  solid  panels 
The  spandrel-fillings  consist  of  vertical  shafts  united  by  cross 
pieces.  The  piers  are  built,  up  to  support  the  roadway-bearers  ; 
they  are  13  feet  thick  at  the  springing  line.  The  entire  width 
of  the  bridge  is  36  feet,  the  carriage-way  occupying  25  feet. 

(F)  In  this  bold  structure,  the  width  of  each  of  the  two  extreme 
bays  is  210  feet.  The  curved  rib  is  composed  of  thirteen  solid 
panels,  each  of  which  is  2£  inches  thick,  and  has  a  rim,  or  flanch 
around  it  about  4  inches  broad.  The  rib  is  6  feet  deep  at  the 
crown  and  8  feet  at  the  spring.  The  spandrel-filling  is  composed 
of  lozenge-shaped  panels  with  vertical  joints ;  they  are  secured 
to  the  back  of  the  curved  rib  and  support  the  roadway-plates. 
The  curved  ribs  are  connected  by  tie-piates  inserted  between  the 
joints  of  the  voussoirs ;  and  they  are  braced  by  feathered  diago- 
nal braces.  The  piers  are  24  feet  thick  at  the  springing  line, 
and  are  built  up  to  the  level  of  the  roadway-plates.  The  width 
of  the  carriage-way  is  25  feet,  and  that  of  each  of  the  footpaths 
7  fed. 

(Ci)  This  bridge  presents  a  very  light  and  elegant  appearance  ; 
the  panels  of  the  curved  rib  being  cast  with  open  curvilinear 
spaces,  which  divide  the  panel  into  several  rectangular-shaped 
figures,  with  solid  sides  and  diagonals.  Each  rib  consists  of 
twelve  panels.  The  depth  of  the  ribs  is  3  feet.  The  thickness 
of  the  two  exterior  ribs  is  2^  inches,  that  of  the  four  interior 
2  inches.  The  ribs  are  connected  by  grated  tie-plates  between 
the  panel-joints,  and  they  abut  against  springing  plates  which 
ire  3  feet  wide  and  4  inches  thick.  The  roadway-bearers  and 
road-plates  are  of  cast  iron.  The  spandrel-filling  is  composed 
of  lozenge-shaped  panels,  the  sides  of  the  lozenges  being  fea- 
thered, and  tapering  from  the  middle  to  the  extremities.  The 
ribs  of  the  bridge-frame  are  connected  and  braced  in  the  usual 
manner.  The  road-bearers  are  laid  lengthwise  upon  the  ribs,  to 
which  they  are  firmly  secured,  and  they  are  covered  with  iron 
road-plates,  upon  which  the  road-covering  rests.  The  free  road 
space  is  24  feet. 

(H)  In  this  structure,  (Figs.    3£,  139,)  the  engineer  has  de 


248 


BRIDGES,  ETC. 


parted  from  the  \isual  form  of  a  circulai   segment  aict     ana 
adopted  an  elliptical  segment.     The  following  summary  <ie&%  Tip- 


Fig.  138 — Represents  a  .ongitudi- 
nal  section  through  a  pier  and  its 
cast  iron  standard  of  Lary  bridge, 
showing  the  connection  of  the 
cast-iron  framing  and  the  stone 
piers. 

A ,  upper  portion  of  pier 

B,  standard. 

C,  panel  of  the  curved  rib. 

D,  lozenge  spandrel-filling. 


tion  is  extracted  from  the  engineer's  published  account  of  tfcis 
work  : — "  The  arrangement  of  the  design  differs  materially  from 
other  works  of  a  similar  nature  :  first,  in  the  masonry  of  the  piers 


Fig.  139 — Represents  a  cross  section  of  Fig 

138  taken  through  the  axis  of  the  pier. 
A,  portion  of  pier. 

a,  a,  iron  standards. 

b,  b,  road-plates  laid  on  the  roadway-bear- 
ers c,  o 

d,  d,  diagonal  braces  of  the  standards. 

e,  roadway-skirting  forming  the  base  of  the 
parapet. 


finishing  at  the  springing  course  of  the  arches ;  secondly,  in  the 
curvilinear  forms  of  the  piers  and  abutments  ;  and  thirdly,  in  the 
employment  of  elliptical  arches. 

"  The  centre  arch  is  100  feet  span,  and  rises  14  feet  6  inches. 


CAST-IRON  BRIDGES.  24$ 

the  thickness  of  the  piers,  where  smallest,  being  11)  feel.  The 
arches  adjoining  the  centre  are  95  feet  span  each,  and  rise  13 
feet  3  inches.  The  piers,  taken  as  before,  are  each  9  feet  6 
inches  thick.  The  extreme  arches  are  each  81  feet  span,  and 
rise  10  feet  6  inches.  The  abutments  are,  in  their  smallest  di 
mensions,  13  feet  thick,  forming  at  the  back  a  strong  arch  abutting 
against  the  return-walls  to  resist  the  horizontal  thrust.  The  ends 
of  the  piers  are  semicircular,  having  a  curvilinear  batter  on  the 
sides  and  ends  formed  with  a  radius  of  35  feet,  and  extending 
upward  from  the  level  of  high  water  to  the  springing  course,  and 
downward  to  the  level  of  the  water  at  the  lowest  ebb.  The 
front  of  the  abutments  have  a  corresponding  batter. 

"  The  roadway  is  24  feet  wide,  supported  by  5  cast-iron  equi- 
distant ribs.  Each  rib  is  2  feet  6  inches  in  depth  at  the  spring- 
ing, and  2  feet  at  the  apex,  by  2  inches  thick,  with  a  top  and 
bottom  flange  of  6  inches  wide  by  2  inches  thick,  and  is  cast  in 
5  pieces ;  their  joints  (which  are  flanged  for  the  purpose)  are 
connected  by  screw-pins  with  tie-plates  equal  in  length  to  the 
width  of  the  roadway,  and  in  depth  and  thickness  to  the  ribs ; 
between  these  meeting-plates  the  ribs  are  connected  by  strong 
feathered  crosses,  or  diagonal  braces,  with  screw-pins  passing 
through  their  flanges  and  the  main  ribs.  The  springing-plates 
are  3  inches  thick,  with  raised  grooves  to  receive  the  ends  of  the 
ribs,  which  have  double  shoulders.  These  plates  are  sunk  flush 
into  the  springing  course  of  the  piers  and  abutments,  which,  with 
the  cordon  and  springing  course,  are  of  granite.  The  pier- 
standards  and  spandrel-fillings  are  feathered  castings,  connected 
transversely  by  diagonal  braces  and  wrought-iron  bars  passing 
through  cast-iron  pipes,  with  bearing-shoulders  for  the  several 
parts  to  abut  against.  The  roadway-bearers  are  7  inches  in 
depth  by  li  thick,  with  a  proportional  top  and  bottom  flange; 
they  are  fastened  to  the  pier-standards  by  screw-pins  through 
sliding  mortises,  whereby  a  due  provision  is  made  for  either  ex 
pansion  or  contraction  of  the  metal ;  the  roadway-plates  are  £  of 
an  inch  thick  by  3  feet  wide,  connected  by  flanges  and  screw- 
pins,  and  projeci  1  foot  over  the  outer  roadway-bearers,  thus 
forming  a  cornice  the  whole  length  of  the  bridge. 

"  The  adoption  of  these  forms  for  the  piers  and  arches,  in  uni- 
son with  the  plan  of  finishing  the  piers  above  the  springing  course 
with  cast  iron  instead  of  masonry,  has,  as  I  had  hoped,  given  a 
degree  of  uniform  lightness  combined  with  strength  to  the  general 
effect,  unobtainable  by  the  usual  form  of  straight-sided  piers  car- 
ried to  the  height  of  the  roadway,  with  flat  segments  of  a  circle 
for  the  arches." 

(I)  The  curved  ribs  of  this  bridge  are  tubular,  the  cross  sec 
tion  of  the  tube  being  an  ellipse,  the  transverse  axis  of  which  is 

32 


850  BRIDGES,  ETC. 

2  feet  6  inches,  and  the  conjugate  about  1  foot  4  inches.  Each 
rib  consists  of  eleven  pieces,  which  are  shaped  and  connected  as 
described  under  the  head  of  Framing.  The  spandrel-fillings  are 
formed  of  contiguous  cast-iron  rings  which  rest  upon  the  ribs, 
and  support  the  longitudinal  roadway-bearers.  The  ribs  are  tied 
and  braced  nearly  in  the  usual  manner.  The  flooring  upon  which 
the  road-covering  is  laid  is  of  timber.  The  piers  are  built  up  to 
receive  the  roadway-bearers. 

The  system  of  M.  Polonceau  presents  a  very  light  and  elegant 
form  of  cast-iron  bridge.  The  inventor  claims  for  it  moie  econo- 
my than  by  the  ordinary  combinations,  and  also  more  lightness 
combined  with  adequate  strength.  It  has  been  objected  to  this 
system  that  it  is  defective  in  rigidity ;  this  the  inventor  seems 
disposed  to  regard  as  an  advantage,  and  has  preferred  the  span- 
drel-filling of  rings  partly  on  this  account,  because  their  elasticity 
is  favorable  to  a  gradual  yielding  and  restoration  of  form  in  the 
parts. 

611.  Effects  of  Temperature  on  stone  and  cast-iron  Bridges 
The  action  of  variations  of  temperature  upon  masses  of  masonry, 
particularly  in  the  coping,  has  already  been  noticed.  The  effect 
of  the  same  action  upon  the  equilibrium  of  arches  was  first  ob- 
served by  M.  Vicat  in  the  stone  bridge  built  by  him  at  Souillac, 
in  the  joints  of  which  periodical  changes  were  found  to  take  place, 
not  only  from  the  ranges  of  temperature  between  the  seasons,  but 
even  daily.  Similar  phenomena  were  also  very  accurately  noted 
by  Mr.  George  Rennie  in  a  stone  bridge  at  Staines. 

From  these  recorded  observations  the  fact  is  conclusively  es- 
tablished, that  the  joints  of  stone  bridges,  both  in  the  arches  and 
spandrels,  are  periodically  affected  by  this  action,  which  must 
consequently  at  times  throw  an  increased  amount  of  pressure 
upon  the  abutments,  but  without,  under  ordinary  circumstances, 
any  danger  to  the  permanent  stability  of  the  structure. 

When  iron  was  first  proposed  to  be  employed  for  bridges,  ob- 
jections were  brought  against,  it  on  the  ground  of  the  effect  of 
changes  of  temperature  upon  this  metal.  The  failure  in  the 
abutments  of  the  iron  bridge  at  Staines  was  imputed  to  this  cause, 
and  like  objections  were  seriously  urged  against  other  structures 
about  to  be  erected  in  England.  To  put  this  matter  at  rest,  ob- 
servations were  very  carefully  made  by  Sir  John  Rennie  upon 
the  arches  of  Southwark  bridge,  built  by  his  father.  From  these 
experiments  it  appears  that  the  mean  rise  of  the  centre  arch  at 
tiie  crown  was  about  —th  of  an  inch  for  each  degree  of  Fahr., 
or  1.25  inches  for  50°  Fahr.  The  change  of  form  and  increase 
of  pressure  arising  from  this  cause  do  not  appear  to  have  affected 
in  any  sensible  degree  the  permanent  stability  either  of  this  struc 
ture,  or  of  any  of  a  like  character  in  Europe. 


SUSPENSION  BRIDGES.  251 


SUSPENSION  BRIDGES. 


612.  The  use  of  flexible  materials,  as  cordage  and  the  like,  tc 
form  a  roadway  over  chasms,  and  narrow  water-courses,  dates 
from  a  very  early  period  ;  and  structures  of  this  character  were 
probably  among  the  first  rude  attempts  of  ingenuity,  before  the 
arts  of  the  carpenter  and  mason  were  sufficiently  advanced  to  be 
made  subservient  to  the  same  ends.  The  idea  of  a  suspended 
roadway,  in  its  simplest  form,  is  one  that  would  naturally  present 
itself  to  the  mind,  and  its  consequent  construction  would  demand 
only  obvious  means  and  but  little  mechanical  contrivance  ;  but 
the  step  from  this  stage  to  the  one  in  which  such  structures  are 
now  found,  supposes  a  veiy  advanced  state  both  of  science  and  of 
its  application  to  the  industrial  arts,  and  we  accordingly  find  that 
bridge  architecture,  under  every  other  guise,  was  brought  to  a 
high  degree  of  perfection  before  the  suspension  bridge,  as  this 
structure  is  now  understood,  was  attempted. 

With  the  exception  of  some  isolated  cases  which,  but  in  the 
material  employed,  differed  little  from  the  first  rude  structures, 
no  recorded  attempt  had  been  made  to  reduce  to  systematic  rules 
the  means  of  suspending  a  roadway  now  in  use,  until  about  the 
year  1801,  when  a  patent  was  taken  out  in  this  country  for  the 
purpose,  by  Mr.  Finlay,  in  which  the  manner  of  hanging  the 
chain  supports,  and  suspending  the  roadway  from  it,  are  speci 
fically  laid  down,  differing,  in  no  very  material  point,  from  tht 
practice  of  the  present  day  in  this  branch  of  bridge  architecture. 
Since  then,  a  number  of  structures  of  this  character  have  been 
erected  both  in  the  United  States  and  in  Europe,  and,  in  son& 
instances,  valleys  and  water-courses  have  been  spanned  by  them 
under  circumstances  which  would  have  baffled  the  engineer's  art 
in  the  employment  of  any  other  means. 

A  suspension  bridge  consists  of  the  supports,  termed  piers, 
from  which  the  suspension  chains  are  hung ;  of  the  anchoring 
masses,  termed  the  abutments,  to  which  the  ends  of  the  suspen- 
sion chains  are  attached ;  of  the  suspension  chains,  termed  the 
main  chains,  from  which  the  roadway  is  suspended  ;  of  the  verti- 
cal rods,  or  chains,  termed  the  saspending-chains,  &c,  which 
connect  the  roadway  with  the  main  chains  ;  and  of  the  roadway. 

613.  As  the  general  principles  upon  which  flexible  supports 
for  structures  should  be  arranged  have  already  been  laid  down 
under  the  head  of  Framing,  nothing  more  will  be  requisite,  undci 
the  present  head,  than  to  add  those  modifications  of  the  applies 
tions  of  these  principles  called  for  by  the  character  of  the  stri"*. 
tures  in  question. 

614.  Bays.  The  natural  water-way  may  be  divided  into  ai»> 
number  of  equal-sized  bays,  depending  on  local  circumstance 


262  3RIDGES,    ETC. 

and  the  comparative  cost  of  high  or  low  piers,  and  that  of  th« 
main  chains,  and  the  suspending-rods. 

A  bridge  with  a  single  bay  of  considerable  width  presents  a 
bolder  and  more  monumental  character,  and  its  stability,  all  other 
things  being  equal,  is  greater,  the  amplitude  from  undulations 
caused  by  a  moveable  load  being  less  than  one  of  several  bays. 

If  two  bays  of  equal  span  are  preferred  to  a  single  one  with 
an  equal  versed  sine,  the  chains  may  be  supported  either  by  a 
single  central  pier,  or  by  three  piers  of  the  same  height.  With 
a  single  pier,  the  structure  will  present  the  appearance  (Fig.  101, 
Art.  538)  of  two  half  curves.  The  tension  on  the  chains  and 
the  horizontal  strain  at  the  top  of  the  pier  will,  in  this  case,  be 
the  same  as  in  that  of  the  full  curve  of  double  the  span  and  the 
same  versed  sine ;  and  twice  as  great  as  in  the  case  of  three 
piers  with  curves  of  equal  span  and  the  same  versed  sine. 

If,  instead  of  a  central  pier  with  two  semi-arches,  two  entire 
arches  be  preferred  for  the  bridge,  then  three  piers  will  be  neces- 
sary, which  need  only  be  half  the  height  of  those  which  a  single 
bay  would  require.  The  tension  on  the  chains  in  this  case  will 
be  only  one  fourth  of  that  upon  the  chains  of  a  bridge  with  a  sin- 
gle bay  of  double  width  ;  and  the  abutments  may  be  made  pro- 
portionally less  strong. 

615.  Piers.  These  are  commonly  masses  of  masonry  in  the 
shape  of  pillars,  or  columns,  that  rest  on  a  common  foundation,  and 
are  usually  connected  at  top.  The  form  given  to  the  pier,  when 
of  stone,  will  depend  in  some  respects  on  the  locality.  'Generally 
it  is  that  of  the  architectural  monument  known  as  the  Triumphal 
Arch ;  an  arched  opening  being  formed  in  the  centre  of  the  mass 
for  the  roadway,  and  sometimes  two  others  of  smaller  dimensions, 
on  each  side  of  the  main  one,  for  approaches  to  the  footpaths  of 
the  bridge. 

Piers  of  a  columnar,  or  of  an  obelisk  form,  have  in  some  in- 
stances been  tried.  They  have  generally  been  found  to  be  want- 
ing in  stiffness,  being  subject  to  vibrations  from  the  action  of  the 
chains  upon  them,  which  in  turn,  from  the  reciprocal  action  upon 
the  chains,  tends  very  much  to  increase  the  amplitude  of  the  vi- 
brations of  the  latter.  These  effects  have  been  observed  to  be 
the  more  sensible  as  the  columnar  piers  are  the  higher  and  more 
slender. 

Cast-iron  piers,  in  the  form  of  columns  connected  at  top  by  an 
entablature,  have  been  tried  with  success,  as  also  have  been 
columnar  piers  of  the  same  material  so  arranged,  with  a  joint  at 
iheir  base,  that  they  can  receive  a  pendulous  motion  at  top  to  ac- 
commodate any  increase  of  tension  upon  either  branch  of  the  chain 
resting  on  them. 

The  dimensions  of  piers  will  depend  upon  their  height  and  the 


SUSPENSION  BRIDGES.  25J 

strain  upon  thjm.  When  built  of  stone,  the  masonry  should  be 
very  carefully  constructed  of  large  blocks  well  bonded,  and  tied 
by  metal  cramps.  The  height  of  the  piers  will  depend  mostly 
on  the  locality.  When  of  the  usual  forms,  they  should  at  least 
be  high  enough  to  admit  the  passage  of  vehicles  under  the  arched 
way  of  the  road. 

616.  Abutments.  The  forms  and  dimensions  of  the  abutments 
will  depend  upon  the  manner  in  which  they  may  be  connected 
with  the  chains.  When  the  locality  will  admit  the  chains  to  be 
anchored  without  deflecting  them  vertically,  the  abutments  may 
be  formed  of  any  heavy  mass  of  rough  masonry,  which,  from  its 
weight,  and  the  manner  in  which  it  is  imbedded,  have  sufficient 
strength  to  resist  the  tension  in  the  direction  of  the  chain.  If  it 
is  found  necessary  to  deflect  the  chains  vertically  to  secure  a  good 
anchoring  point,  it  will  also  generally  be  necessary  to  build  a  mass 
of  masonry  of  an  arched  form  at  the  point  where  the  deflection 
takes  place,  which,  to  present  sufficient  strength  to  resist  the 
pressure  caused  by  the  resultant  of  the  tension  on  the  two 
branches  of  the  chain,  should  be  made  of  heavy  blocks  of  cut 
stone  well  bonded.  If  the  abutments  are  not  too  far  from  the 
foundations  of  the  piers,  it  will  be  well  to  connect  the  two,  in 
order  to  give  additional  resistance  to  the  anchoring  points. 

617.  Main  Chains,  &c.  The  suspending  curves,  or  arches, 
may  be  made  of  chains  formed  of  flat,  or  round  iron,  or  may  con 
sist  of  wire  cables  constructed  in  the  usual  manner. 

The  main  chains  of  the  earlier  suspension  bridges  were  formed 
of  long  links  of  round  iron  made  in  the  usual  way ;  but,  indepen- 
dently of  the  greater  expense  of  these  chains,  they  were  found  to  be 
liable  to  defects  of  welding,  and  the  links,  when  long,  were  apt  to 
become  misshapen  under  a  great  strain,  and  required  to  be  slayed 
to  preserve  their  form.  Chains  formed  of  long  links  of  flat  bars, 
usually  connected  hy  shorter  ones,  as  coupling  links,  have  on 
these  accounts  superseded  those  of  the  ordinary  oval-shaped 
links. 

The  breadth  of  the  chains  has  generally  been  made  uniform ; 
but  in  some  recent  bridges  erected  in  England  by  Mr.  Dredge, 
the  chains  are  made  to  increase  uniformly  in  oreadth,  by  increas- 
ing the  number  of  bars  in  a  link,  from  the  centre  to  the  points  of 
suspension,  in  addition  to  this  change  in  the  form  of  the  main 
chains,  Mr.  Dredge  places  the  suspending  chains  in  a  vertical 
plane  parallel  to  the  axis  of  the  bridge,  but  obliquely  to  the  hori- 
zon, inclining  each  way  from  the  points  of  suspension  towards 
the  centre  of  the  curve.  From  experiments,  it  appears  that  a 
trery  considerable  increase  of  strength,  for  the  same  amount  of 
material,  is  given  by  these  modifications. 

The  number  and  disposition  of  the  chains  will  depend  upon  th« 


854  BRIDGES,  ETC. 

strain  to  be  borne  and  the  arrangement  of  the  roadway  and  foot 
paths.  For  a  single  carriage-way  the  main  chains  are  disposed 
on  each  side,  leaving  the  requisite  width  of  the  carriage-way  be- 
tween them.  Should  the  weight  to  be  bcrne  be  so  great  that  the 
number  of  bars  in  a  link  would  give  such  breadth  to  the  chain  as 
to  require  a  considerable  addition  to  the  breadth  of  the  piers,  two 
or  more  chains  must  be  employed,  and  these  should  be  suspended 
one  immediately  below  the  other.  It  lias  been  suggested  that 
their  distance  apart  should  be  such  that  the  shadow  from  the 
chain  above  upon  that  beneath  should  not  prevent  the  action  of 
the  sun's  rays,  in  evaporating  any  moisture  that  may  lodge  in  the 
articulations  of  the  links,  and  also  to  preserve  an  equable  temper- 
ature in  all  the  chains.  If  there  are  two  carriage-ways,  with 
footpaths,  any  arrangement  of  the  chains  may  be  adopted,  simi- 
lar to  those  already  pointed  out  for  the  ribs  of  wooden  bridges 
under  like  circumstances  ;  care  being  taken  that  the  strength  of 
the  chains  be  proportioned  to  the  strain  upon  them,  and  that  they 
be  placed  so  far  asunder,  that  in  violent  oscillations  from  high 
winds  they  may  not  come  into  collision. 

Some  of  the  links  of  the  main  chains  should  be  arranged  witl 
adjusting  screws,  or  with  keys,  to  bring  the  chains  to  the  proper 
degree  of  curvature  when  set  up. 

The  chains  may  either  be  attached  to,  or  pass  over  a  moveable 
cast-iron  saddle,  seated  on  rollers  on  the  top  of  the  piers,  so  that 
it  will  allow  of  sufficient  horizontal  displacement  to  permit  the 
chains  to  accommodate  themselves  to  the  effects  of  a  moveable 
load  on  the  roadway.  The  same  ends  may  be  attained  by  attach- 
ing the  chains  to  a  pendulum  bar  suspended  from  the  top  of  the 
pier. 

The  chains  are  firmly  connected  with  the  abutments,  by  being 
attached  to  anchoring  masses  of  cast  iron,  arranged  in  a  suitable 
manner  to  receive  and  secure  the  ends  of  the  chains,  which  are 
carefully  imbedded  in  the  masonry  of  the  abutments.  These 
points,  when  under  ground,  should  be  so  placed  that  they  can  be 
visited  and  examined  from  time  to  time. 

618.  Suspending  Chains.  The  suspending-rods,  or  chains, 
should  be  attached  to  such  points  of  the  main  chains  and  the 
roadway-bearers,  as  to  distribute  the  load  uniformly  over  the 
main  chains,  and  to  prevent  their  being  broken  or  twisted  off 
dv  the  oscillations  of  the  bridge  from  winds,  or  moveable  loads. 
They  should  be  connected  by  suitably-arranged  articulations,  with 
a  saddle  piece  bearing  upon  the  back  of  the  main  chain,  and  at 
bottom  with  the  stirrup  that  embraces  the  roadway-bearers. 

The  suspending-chains  are  usually  hung  vertically.  In  some 
recent  bridges  they  have  been  inclined  inward  to  give  more  stiff 
dcss  to  the  system. 


SUSPENSION   BRIDGES  26b 

619.  Roadway.  Transversal  roadway-bearers  are  attached 
to  the  suspending-chains,  upon  which  a  flooring  of  timber  is  laid 
for  the  roadway.  The  roadway-bearers,  in  some  instances,  have 
been  made  of  wrought  iron,  but  timber  is  now  generally  preferred 
for  these  pieces.  Diagonal  ties  of  wrought  iron  are  placed  hori- 
zontally between  the  roadway-bearers  to  brace  the  frame-work. 

The  parapet  may  be  formed  in  the  usual  style  either  of  wrought 
iron,  or  of  timber,  or  of  a  combination  of  cast  iron  and  timber 
Timber  alone,  or  in  combination  with  cast  iron,  is  now  preferred 
for  the  parapets  ;  as  observation  has  shown  that  the  stiffness  given 
to  the  roadway  by  a  strongly-trussed  timber  parapet  limits  the 
amplitude  of  the  undulations  caused  by  violent  winds,  and  secures 
the  structure  from  danger 

In  some  of  the  more  recent  suspension  bridges,  a  trussed 
frame,  similar  to  the  parapet,  has  been  continued  below  the  level 
of  the  roadway,  for  the  purpose  of  giving  greater  security  to  the 
structure  asainst  the  action  of  high  winds. 

When  the  roadway  is  above  the  chains,  any  requisite  number 
of  single  chains  may  be  placed  for  its  support.  Frames  formed 
of  vertical  beams  of  timber,  or  of  columns  of  cast  iron  united  by 
diagonal  braces,  rest  upon  the  chains,  and  support  the  roadway- 
bearers  placed  either  transversely,  or  longitudinally. 

620.  Vibrations.  The  undulatory  or  vibratory  motions  of 
suspension  bridges,  caused  by  the  action  of  high  winds,  or  move- 
able loads,  should  be  reduced  to  the  smallest  practicable  amount, 
by  a  suitable  arrangement  of  bracing  for  the  roadway-timbers  and 
parapet,  and  by  chain-stays  attached  to  the  roadway  and  to  the 
basements  of  the  piers,  or  to  fixed  points  on  the  banks  whenever 
they  can  be  obtained. 

Calculation  and  experience  show  that  the  vibrations  caused  by 
a  moveable  load  decrease  in  amplitude  as  the  span  increases, 
and,  for  the  same  span,  as  the  versed  sine  decreases.  The 
heavier  the  roadway,  also,  all  other  things  being  the  same,  the 
smaller  will  be  the  amplitude  of  the  vibrations  caused  by  a  move- 
able load,  and  the  less  will  be  their  effect  in  changing  the  form 
of  the  bridge. 

The  vibrations  caused  by  a  moveable  load  seldom  affect  the 
bridge  in  a  hurtful  degree,  owing  to  the  elasticity  of  the  system, 
unless  they  recur  periodically,  as  in  the  passage  of  a  body  of 
soldiers  with  a  cadenced  march.  Serious  accidents  have  been 
occasioned  in  this  way ;  also  by  the  passage  of  cattle,  and  by 
the  sudden  rush  of  a  crowd  from  one  side  of  the  bridge  to  the 
other.  Injuries  of  this  character  can  on.y  be  guarded  against  by 
\  proper  system  of  police  regulations. 

Chain-stays  may  either  be  attached  to  some  point  of  tne  road 
way.  and  to  fixed  points  beneath  it,  or  else  they  may  be  m  the 


266  BRIDGES,  ETC. 

form  of  a  reversed  curve  below  the  roadway.  The  former  is  the 
more  efficacious,  but  it  causes  the  bridge  to  bend  in  a  disagree- 
able manner  at  the  point  where  the  stay  is  attached,  when  the 
action  of  a  moveable  load  causes  the  main  chains  to  rise.  The 
more  oblique  the  stays,  the  longer,  more  expensive,  and  les« 
effective  they  become.  Stays  in  the  form  of  a  reversed  curvt 
presejve  better  the  shape  of  the  roadway  under  the  action  of  a 
moveable  load,  but  they  are  less  effective  in  preventing  vibrations 
than  the  simple  stay.  Neither  of  these  methods  is  very  service- 
able, except  in  narrow  spans.  In  wide  spans,  variations  of  tem- 
perature cause  considerable  changes  in  the  length  of  the  stays, 
which  makes  them  act  unequally  upon  the  roadway ;  this  is  par- 
ticularly the  case  with  the  reversed  curve.  Both  kinds  should 
be  arranged  with  adjusting  screws,  to  accommodate  their  length 
to  the  more  extreme  variations  of  temperature. 
.  Engineers,  at  present,  generally  agree  that  the  most  efficacious 
means  of  limiting  the  amplitude,  and  the  consequent  injurious 
effects  of  undulations,  consists  in  a  strong  combination  of  the 
roadway-timbers  and  flooring,  stiffened  by  a  trussed  parapet  of 
timber  above  the  roadway,  and  in  some  cases  in  extending  the 
frame-work  of  the  parapet  below  it.  These  combinations  pre- 
sent, in  appearance,  and  reality,  two  or  more  open-built  beams, 
as  circumstances  may  demand,  placed  parallel  to  each  other,  and 
strongly  connected  and  braced  by  the  frame-work  of  the  road- 
way, which  are  supported  at  intermediate  points  by  the  suspend- 
ing-rods,  or  chains.  The  method  of  placing  the  roadway-framing 
at  the  central  line  of  the  open-built  beams  presents  the  advantage 
of  introducing  vertical  diagonal  braces,  or  ties  between  the  beams 
beneath  the  roadway-frame.  The  main  objection  to  these  com- 
binations is  the  increased  tension  thrown  upon  the  chains  from 
the  greater  weight  of  the  frame-work.  This  increase  of  tension, 
however,  provided  it  be  kept  within  proper  limits,  so  far  from 
being  injurious,  adds  to  the  stability  and  security  of  the  bridge, 
both  from  the  effects  of  undulations  and  of  vibrations  from  shocks 

As  a  farther  security  to  the  stability  of  the  structure,  the  frame 
work  of  the  roadway  should  be  firmly  attached  at  the  two  extre 
mities  to  the  basements  of  the  piers. 

621.  Preservative  means.  To  preserve  the  chains  from  oxi 
dation  on  the  surface,  and  from  rain  or  dews  which  may  lodge  in 
the  articulations,  they  should  receive  several  coats  of  minium  or 
of  some  other  preparation  impervious  to  water,  and  this  sbculd 
be  renewed  from  time  to  time,  and  the  forms  of  all  the  parts 
should  be  the  most  suitable  to  allow  the  free  escape  of  moisture. 

Wires  for  cables  can  be  preserved  from  oxidation,  until  they 
are  made  into  ropes,  by  keeping  them  immersed  in  some  alkalint 
solution.     Before  making  them  into  ropes  they  should  be  dipped 


SUSPENSION  BRIDGES.  257 

several  times  in  I  oiling  linseed  oil,  prepared  by  previously  boil- 
ing it  with  a  small  portion  of  litharge  and  lampblack.  The  cables 
should  receive  a  thick  coating  of  the  same  preparation  before 
they  arc  put  up,  and  finally  be  painted  with  white  lead  paint,  both 
as  a  preservative  means,  and  to  show  any  incipient  oxidation,  as 
the  rust  will  be  detected  by  its  discoloring  the  paint. 

622.  Proofs  of  Suspension  Bridges.  From  the  many  grave 
accidents,  accompanied  by  serious  loss  of  life,  which  have  taken 
place  in  suspension  bridges,  it  is  highly  desirable  that  some  trial - 
proof  should  be  made  before  opening  such  bridges  to  the  public, 
and  that,  moreover,  strict  police  regulations  should  be  adopted 
and  enforced,  with  respect  to  them,  to  guard  against  the  recur- 
rence of  such  disasters  as  have  several  times  taken  place  in  Eng 
land,  from  the  assemblage  of  a  crowd  upon  the  bridge.  In 
France,  and  on  the  continent  generally,  where  one  of  the  impor- 
tant duties  of  the  public  police  is  to  watch  over  the  safety  of  life, 
under  such  circumstances,  regulations  of  this  character  are  rigidly 
enforced.  The  trial-proof  enacted  in  France  for  suspension 
bridges,  before  they  are  thrown  open  for  travel,  is  about  40  lbs. 
to  each  superficial  foot  of  roadway  in  addition  to  the  permanent 
weight  of  the  bridge.  This  proof  is  at  first  reduced  to  one  half, 
in  order  not  to  injure  the  masonry  of  the  points  of  support  during 
the  green  condition  of  the  mortar.  It  is  made  by  distributing 
over  the  road  surface  any  convenient  weighty  material,  as  bricks, 
pigs  of  iron,  bags  of  earth,  &c.  Besides  this  after-trial,  each 
element  of  the  main  chains  should  be  subjected  to  a  special  proof 
to  prevent  the  introduction  of  unsound  parts  into  the  system. 
This  precaution  will  not  be  necessary  for  the  wire  of  a  cable,  as 
the  process  of  drawing  alone  is  a  good  test.  Some  of  the  coils 
tested  will  be  a  guarantee  for  the  whole. 

From  experiments  made  at  Geneva  by  Colonel  Dufour,  one  of 
the  earliest  and  most  successful  constructors  of  suspension  bridges 
on  the  Continent,  it  appears  that  wrought  bar  iron  can  sustain 
without  danger  of  rupture  a  shock  arising  from  a  weight  of  44 
lbs.  raised  to  a  height  of  3.28  feet  on  each,  .0015dths  of  an  inch 
of  cross  section,  when  the  bar  is  strained  by  a  weight  equal  to 
one  third  of  its  breaking  weight ;  and  he  concludes  that  no  ap- 
prehension need  be  entertained  of  injury  to  a  bridge  from  shocks, 
caused  by  the  ordinary  transit  upon  it,  which  has  been  subjectea 
to  the  usual  trial  of  a  dead  weight ;  and  that  the  safety,  in  this 
respect,  is  the  greater  as  the  bridge  is  longer,  since  the  elasticity 
of  the  system  is  the  best  preservative  from  accidents  due  to  such 
causes. 

623.  Durability.  Time  is  the  true  test  of  the  durability  o 
the  structures  under  consideration.  So  far  as  experience  goes 
there  seems  to  be  no  reason  to  assign  less  durability  to  suspen 

33 


258  BRIDGES,  ETC. 

sion  than  to  cast-iron,  or  even  stone  bridges,  if  their  repairs  and 
the  proper  means  of  preserving  them  from  decay  are  attended  to. 
Doubts  have  been  expressed  as  to  the  durability  of  wire  cables, 
but  these  seem  to  have  been  set  at  rest  by  the  trials  and  exami- 
nations to  which  a  bridge  of  this  kind,  erected  by  Colonel  Dufour, 
at  Geneva,  was  subjected  by  him  after  twenty  years  service.  It 
was  found  that  the  undulations  were  greater  than  when  the  bridge 
was  first  erected,  owing  to  the  shrinking  of  the  roadway-frame  ; 
but  the  main  cables,  and  suspending-ropes,  even  at  the  loops  in 
contact  with  the  timber,  proved  to  be  as  sound  as  when  first  put 
up,  and  free  from  oxidation ;  and  the  whole  bridge  stood  another 
very  severe  proof  without  injury. 

624.  The  following  succinct  descriptions  of  the  principal  ele- 
ments of  some  of  the  most,  celebrated  suspension  bridges  of 
chains,  and  wire  cables,  of  remarkable  span,  are  taken  from  va- 
rious published  accounts. 

Bridge  over  the  Tweed  near  Bei-wick.  This  is  the  first  large 
suspension  bridge  erected  in  Great  Britain.  It  was  constructed 
upon  the  plans  of  Capt.  Brown,  who  took  out  a  patent  for  the 
principles  of  its  construction. 

Span         ....       449  feet 
Versed  sine       .         .         .         30    " 
Number  of  main  chains  1 2,  six  being  placed  on  each  side  of  the 

.roadway,  in   three  ranges,  of  two  chains  each,  above  each 

other. 

The  chains  are  composed  of  long  links  of  round  iron,  2  inches 
in  diameter,  and  are  15  feet  long.  They  are  connected  by  coupling 
links  of  round  iron,  \\  inch  diameter,  and  about  7  inches  long,  by 
means  of  coupling  bolls. 

The  roadway  is  korne  by  suspending-rods  of  round  iron,  which 
are  attached  alternately  to  the  three  ranges  of  chains.  The  road- 
way-bearers are  of  timber,  and  are  laid  upon  longitudinal  bars 
of  wrought  iron,  which  are  attached  to  the  suspension  rods. 

Menai  Bridge,  erected  after  the  designs  of  Mr.  Telford. 
Opened  in  1826. 

Span       ....       579.8  feet. 
Versed  sine     .         .  43        " 

Number  of  main  chains  16,  arranged  in  sets  of  4  eai:h,  vertically 

above  each  other. 
Number  of  bars  in  each  link  5. 
Length  of  links  10  feet. 

Breadth  of  each  bar  3}  inches  ;  depth  1  inch. 
Coupling  links  16  inches  long,  8  inches  broad,  and  1  inch  deep 
Coupling  bolts  3  inches  in  diameter. 
Total  area  of  cross  section  of  the  main  chain,  260  square  inches 

The  main  chains  are  fastened  to  their  abutments  by  anchoring 


SUSPENSION  BRIDGES.  25§ 

nolts  9  feet  long  and  6  inches  in  diameter,  which  are  secured  ir 
cast-iron  grooves.  The  abutments,  which  are  underground  and 
reached  by  suitable  tunnels,  are  the  solid  rock. 

Upon  the  tops  of  the  piers  are  cast-iron  saddles,  upon  which 
the  main  chains  rest.  The  base  of  the  saddle,  which  is  fitted 
with  grooves  to  receive  them,  rests  upon  iron  rollers  placed  on  a 
convex  cylindrical  bed  of  cast  iron,  shaped  like  the  bottom  of  the 
base  of  the  saddle,  to  admit  of  a  slight  displacement  of  the  chains 
from  moveable  loads,  or  changes  of  temperature. 

The  roadway  is  divided  into  two  carriage-ways,  each  12  feel 
wide,  and  a  footpath  4  feet  wide  between  them.  The  roadway- 
framing  consists  of  444  wrought-iron  roadway-bearers,  3|  inches 
deep  and  ^  inch  thick,  which  are  supported  at  the  centre  points 
of  each  of  the  carriage-ways  by  an  inverted  truss,  consisting 
of  two  bent  iron  ties  which  support  a  vertical  bar  placed  under 
the  roadway-bars  at  the  points  just  mentioned.  The  platform 
of  the  roadway  is  formed  of  two  thicknesses  of  plank.  The 
first,  3  inches  thick,  is  laid  on  the  roadway-bearers  and  fastened 
to  them.  This  is  covered  by  a  coating  of  patent  felt  soaked  in 
boiling  tar.  The  second  is  two  inches  thick  and  spiked  to  the 
first. 

The  roadway  is  suspended  by  articulated  rods  attached  to 
stirrups  on  the  roadway-bearers  and  to  the  coupling  bolts  of  the 
main  chains. 

The  piers  are  152  feet  high  above  the  high- water  level.  They 
have  an  arched  opening  leading  to  the  roadway,  and  the  masses 
on  the  sides  of  the  arch  are  built  hollow,  with  a  cross-tie  partition 
wall  between  the  exterior  main  walls. 

The  parapet  is  of  wrought-iron  vertical  and  parallel  bars  con- 
nected by  a  network. 

This  bridge  was  seriously  injured  by  a  violent  gale,  which  gave 
so  great  an  oscillation  to  the  main  chains  that  they  were  dashed 
against  each  other,  and  the  rivet-heads  of  the  bolts  were  broken 
off.  To  provide  against  similar  accidents,  a  frame-work  of  cast 
iron  tubes,  connected  by  diagonal  pieces,  was  fastened  at  inter- 
vals between  the  main  chains,  by  cross  ties  of  wrought-iron  rods, 
which  passed  through  the  tubes,  and  were  firmly  connected  with 
the  exterior  chains.  Subsequently  to  this  addition,  a  number  of 
strong  limber  roadway-bearers  were  fastened  at  intervals  to  those 
of  iron,  as  the  iron  roadway-bearers  were  found  to  have  been 
bent,  and  in  some  instances  broken,  by  the  undulatory  motion  of 
the  bridge  in  heavy  gales. 

The  total  suspending  weight  of  this  bridge,  including  the  main 
chains,  roadway,  and  all  accessories,  is  stated  at  643  tons,  15  j 
cwt 

The  Fribourg  bridge  of  wire  thrown  across  the  valley  of  ih« 


260  BRIDGET,  ETC 

Sarine,  opposite  Fribourg,  was  erected  in  1832  by  M.  Cha  ey,  a 
French  engineer. 

Span         .         .  870.32  feet. 

Versed  sine       .         .         63.26    " 

There  are  4  main  cables,  two  on  each  side  of  the  road,  of  the 
same  elevation,  and  about  1}  inch  asunder.  Each  cable  is  com- 
posed of  1056  wires,  each  about  0.118  inch  in  diameter,  which 
are  firmly  connected  and  brought  to  a  cylindrical  shape  by  a  spiral 
wire  wrapping.  The  diameter  of  the  cable  varies  from  5  to  5} 
inches.  The  cables  pass  over  3  fixed  pulleys  on  the  top  of  the 
piers,  upon  which  they  are  spread  out  without  ligatures,  and  are 
each  attached  to  two  other  cables  of  half  their  diameter  which 
are  anchored  at  some  distance  from  the  piers,  in  vertical  pits, 
passing  over  a  fixed  pulley  where  they  enter  the  mouth  of  the 
pit. 

The  suspending-ropes  are  of  wire  a  size  smaller  than  that  used 
for  the  cables.  Their  diameter  is  nearly  1  inch.  They  are 
formed  with  a  loop  at  each  end,  fastened  around  a  crupper-shaped 
piece  of  cast  iron,  that  forms  an  eye  to  connect  the  rope  with  the 
hook  of  the  stirrup  affixed  to  the  roadway-bearers,  and  to  a  saddle- 
piece  of  wrought  iron,  for  each  rope,  that  rests  on  the  two  main 
cables. 

The  roadway-bearers  are  of  timber,  being  deeper  in  the  centre 
than  at  the  two  ends,  the  top  surface  being  curved  to  conform  to 
a  slight  transverse  curvature  given  to  the  surface  of  the  carriage- 
way ;  they  are  placed  about  5  feet  between  their  centre  lines, 
every  fourth  one  projecting  about  3  feet  beyond  the  ends  of  the 
others,  to  receive  an  oblique  wrought-iron  stay  to  maintain  the 
parapet  in  its  vertical  position.  The  carriage-way,  which  is  about 
15}  feet  wide,  is  formed  of  two  thicknesses  of  plank.  The  foot- 
paths, which  are  6  feet  wide,  are  raised  above  the  surface  of  the 
carriage-way,  and  rest  upon  longitudinal  beams  of  large  dimen- 
sions, the  inner  one  of  which  is  firmly  secured  to  the  roadway- 
bearers  by  stirrups  which  embrace  them,  and  the  exterior  one  is 
fastened  to  the  same  pieces  by  long  screw-bolts,  which  pass 
through  the  top  rail  of  the  parapet.  The  roadway  has  a  slight 
curvature  from  the  centre  to  the  two  extremities,  along  the  axis ; 
the  centre  point  being  from  18  inches  to  about  3  feet  higher  than 
the  ends,  according  to  the  variations  of  temperature.  The  main 
cables  at  the  centre  are  brought  dow  i  nearly  in  contact  with  the 
roadway-timbers. 

The  parapet  is  an  open-built  beam,  consisting  of  a  top  rail,  the 
bottom  rail  being  the  longitudinal  exterior  beam  of  the  footpath, 
and  of  diagonal  pieces  which  are  mortised  into  the  two  rails  ;  the 
whole  being  secured  by  the  iron  bolts  that  pass  through  the  road 
wa) -bearers  and  the   op  rail.     This  combination  of  the  parapet 


SUSPENSION  BRIDGES  261 

with  the  inclination  towards  the  axis  of  the  roadway  given  to  the 
suspending-ropes,  gives  g:  rat  stiffness  to  the  roadway,  and  coun- 
teracts both  lateral  oscillations,  and  longitudinal  undulations. 

The  piers  consist  of  two  pillars  of  solid  masonry,  about  66  fee 
high  above  the  level  of  the  roadway,  which  are  united,  at  about 
H'S  feet  above  the  same  level,  by  a  full  centre  arch,  having  a  span 
of  nearly  20  feet,  and  which  forms  the  top  of  the  gateway  leading 
to  the  bridge. 

Hungerford  and  Lambeth  bridge,  erected  over  the  Thames 
upon  the  plans  of  Mr.  Brunei. 

This  bridge,  designed  for  foot-passengers  only,  has  the  widest 
span  of  any  chain  bridge  erected  up  to  this  period. 
Span         .         .  676i  feet. 

Versed  sine       .         .         50       " 

The  main  chains  are  4  in  number,  two  being  placed  on  each 
side  ( f  the  bridge,  one  above  the  other.  These  chains  are  formed 
entirely*of  long  links  of  flat  bars  ;  the  links  near  the  centre  of  the 
curve  having  alternately  ten  and  eleven  bars  in  each,  and  those 
near  the  piers  alternately  eleven  and  twelve  bars.  The  bars  are 
24  feet  long,  7  inches  in  depth,  and  1  inch  thick.  They  are 
connected  by  coupling-bolts,  4f  inches  in  diameter,  which  are 
secured  at  each  end  by  cast-iron  nuts,  8  inches  in  diameter,  and 
2f  inches  thick.  The  extremity  of  each  chain  is  connected  with 
a  cast-iron  saddle-piece,  by  bolts  which  pass  through  the  vertical 
ribs  of  the  saddle-piece,  of  which  there  are  15.  The  bottom  of 
the  saddle  rests  on  50  friction-rollers,  which  are  laid  on  a  firm 
horizontal  bed  of  cast  iron.  The  saddle  can  move  18  inches 
horizontally,  either  way  from  the  centre,  and  thus  compensate 
for  any  inequality  of  strain  on  the  main  chains,  either  from  a  load, 
or  from  variations  of  temperature. 

The  side  main-chains  are  attached  in  like  manner  to  the  sad- 
dle, and  anchored  at  the  other  extremity  in  an  abutment  of  brick- 
work.    The  anchorage   (Fig.  140)  is  arranged  by  passing  the 


Fie.  140— Shows  the  manner  in  which  the 

side  main-chains  are  anchored. 
A,  inclined  shaft  for  the  chains  leading  to 

the  arched  chamber  H  of  the  anchorage. 
a,  a,  two  main-chains,  passed  through  the 

cast-iron  holding-plate  b  and  fastened 

behind  it  by  keys, 
c,  c,  cross  sections  of  the  cast-iron  girdsn 

which  retain  b. 


cnains  through  a  strong  cast-iron  plate,  and  securing  the  ends  of 


262 


BRIDGES,  ETC. 


he  bars  by  keys.  The  anchoring-plate  is  retained  in  its  place 
oy  two  strong  cast-iron  beams,  against  which  the  strain  upon  the 
plate  is  thrown. 

The  suspend ing-rods  (Fig.  141)  are  connected  with  both  the 


Fig.  141 — Shows  an  eleya- 
tion  M  ami  cross  section 
N  of  the  connection  be- 
tween the  main-chaira 
and  suspending-rods. 

a,  a,  upper  main-chain. 

i,  b,  joint  ot'  lower  main- 
chain. 

c,  suspending-rod  with  a 
forked  head  to  receive  the 
plate  d,  hung  by  stirrup- 
straps  e  and/,  respective- 
ly, to  the  coupling-bolt  of 
the  links  and  to  the  iwc 
bolts#,  fastened  tothe  sad- 
dle h  on  top  of  the  upi>ei 
main-chain.   9 


upper  and  lower  main-chains  ;  to  the  upper  by  a  saddle-piece 
and  bolts,  and  to  the  coupling-bolt  of  the  lower  by  an  arrange- 
ment of  articulations,  which  allows  an  easy  play  to  the  rods  ;  at 
bottom  (Fig.  142)  they  are  connected  by  a  joint  with  a  bolt  that 
fastens  firmly  the  roadway-timbers. 


Fig.  J  42 — Shows  an  elevation  of  the  roadway-timbers. 

a,  bottom  longitudinal  beam. 

b,  b,  roadway-bearers  in  pairs. 

c,  platform.     - 

a,  top  longitudinal  beam  forming  the  bottom  rail  of  the  para- 
pet. 

e,  bolt,  with  a  forked  head  to  receive  the  end  of  the  suspending- 
rod,  which  is  keyed  beneath  and  secures  the  beams,  &c. 

g,  wrought-iron  horizontal  diagonal  ties. 


The  roadway-timbers  consist  of  a  strong  longitudinal  bottom 
beam,  upon  which  the  roadway-bearers  are  notched  ;  these  last 
pieces  are  in  pairs,  the  two  being  so  far  apart  that  the  bolts  con- 
necting with  the  suspending-rods  by  a  forked  head  can  pass  be 
tween  them  ;  the  flooring-plank  is  laid  upon  the  roadway-bearers  ; 
and  a  top  longitudinal  beam,  which  forms  the  bottom  rail  of  the 
parapet,  is  secured  to  the  bottom  beam  by  the  connecting  bolt 
Wrought-iron  diagonal  ties  are  placed  horizontally  below  the 
flooring,  to  brace  the  whole  of  the  timbers  beneath. 

The  roadway  is  14  feet  wide.     It  slopes  from  the  centre  pohi 


SUSPENSION  BRIDGES.  263 

along  the  axis  to  the  extremities,  being  4  feet  higher  in  the  centre 
than  at  the  two  last  points. 

The  piers  are  in  the  form  of  towers,  resembling  the  Italian 
belfry.  They  are  of  brick,  80  feet  high,  and  so  constructed  and 
combined  with  the  top  saddles,  that  they  have  to  sustain  no  oth&i 
strain  than  the  vertical  pressure  from  the  main-chains. 

The  whole  weight  of  the  structure,  with  an  additional  load  of 
100  lbs.  per  square  foot  of  the  roadway,  would  throw  about  100C 
Ions  on  each  pier.  The  tension  on  the  chains  from  this  load  is 
calculated  at  about  1480  tons ;  while  the  strain  they  can  beai 
without  impairing  their  strength  is  about  5000  tons. 

Monongahela  wire  Bridge.  This  bridge,  erected  at  Pittsburgh, 
Penn.,  upon  plans,  and  under  the  superintendence  of  Mr.  Roc- 
bling,  has  8  bays,  varying  between  188  and  190  feet  in  width.  It 
is  one  of  the  more  recent  of  these  structures  in  the  United  States. 

The  roadway  of  each  bay  is  supported  by  two  wire  cables,  of 
4|  inches  in  diameter,  and  by  diagonal  stays  of  wire  rope,  at 
tached  to  the  same  point  of  suspension  as  the  cables,  and  con 
necting  with  different  points  of  the  roadway-timbers.  The  ends 
of  the  cables  of  each  bay  arc  attached  to  pendulum-bars,  by 
means  of  two  oblique  arms,  which  are  united  by  joints  to  the 
pendulum-bars.  These  bars  are  suspended  from  the  top  of  4 
cast-iron  columns,  inclining  inwards  at  lop,  which  are  there  firmly 
united  to  each  other  ;  and,  at  bottom,  anchored  to  the  top  of  a 
stone  pier  built  up  to  the  level  of  the  roadway-timbers.  The 
side  columns  of  each  frame  arc  connected  throughout  by  an  open 
lozenge-work  of  cast  iron.  The  front  columns  have  a  like  con- 
nection, leaving  a  sufficient  height  **f  passage-way  for  foot-pas- 
sengers, j 

The  frame-work  of  4  columns  on  each  side  is  firmly  connected 
at  top  by  cast-iron  beams,  in  the  form  of  an  entablature.  A  car- 
riage-way is  left  between  the  two  frames,  and  a  footpath  between 
•  he  two  columns  forming  the  fronts  of  each  frame. 

The  points  of  suspension  of  the  cables  are  over  the  centre  line 
of  the  footpaths  ;  and  the  cables  are  inclined  so  far  inward  that 
the  centre  point  of  the  curve  is  attached  just  outside  of  the  car- 
riage-way. The  suspending-ropes  have  a  like  inward  inclination, 
the  object  in  both  cases  being  to  add  stiffness  to  the  system,  and 
diminish  lateral  oscillations. 

The  roadway  consists  of  a  carriage-way  22  feet  wide,  and  two 
footpaths  each  5  feet  wide.  The  roadway-bearers  are  transversal 
beams  in  pairs,  35  feet  long,  15  inches  deep,  and  4|  inches  wide 
They  are  attached  to  the  suspending-ropes.  The  flooring  con 
sists  of  2|  inch  plank,  laid  longitudinally  over  the  entire  roadway- 
surface  ;  and  of  a  second  thickness  of  2^  inch  oak  plank  laid 
transversely  o-  er  the  carriage-way. 


264  BRIDGES,  ETC. 

The  parapet,  which  is  on  the  principle  of  Town's  lattice,  ex- 
tends so  far  below  the  roadway-bearers  that  they  rest  and  arc 
notched  on  the  lowest  chord  of  the  lattice.  A  second  chord  em 
braces  them  on  top,  and  finally  a  third  chord  completes  the  lattice 
at  top.  The  object  of  adopting  tins  form  of  parapet  was  to  in- 
crease the  resistance  of  the  roadway  to  undulations. 

MOVEABLE  BRIDGES 

624.  The  term  moveable  bridge  is  commonly  applied  to  a 
platform  supported  by  a  frame-work  of  timber,  or  of  cast  iron, 
by  means  of  which  a  communication  can  be  formed  or  inter- 
rupted at  pleasure,  between  any  two  points  of  a  fixed  bridge,  or 
over  any  narrow  water-way.  These  bridges  are  generally  de- 
nominated draw-bridges,  but  this  term  is  now,  for  the  most  part, 
confined  to  those  moveable  bridges  which  can  be  raised  or  low- 
ered by  means  of  a  horizontal  axis,  placed  either  at  one  extremity 
of  the  platform,  or  at  some  intermediate  point  between  the  two 
ends,  and  a  counterpoise  which  is  so  connected  with  the  platform 
in  either  case,  that  the  bridge  can  be  easily  manoeuvred  by  a 
small  power  acting  through  the  intermedium  of  some  suitable 
mechanism  applied  to  the  counterpoise.  The  term  turning  or 
swinging  bridge  is  used  when  the  bridge  is  arranged  to  turn 
horizontally  around  a  vertical  axis  placed  at  a  point  between  its 
two  ends,  so  that  the  parts  on  each  side  of  the  axis  balance  each 
other ;  and  the  term  rolling  bridge  is  applied  when  the  bridge 
resting  upon  rollers  can  be  shoved  forward  or  backward  horizon- 
tally, to  open  or  interrupt  the  passage. 

To  the  above  may  be  added  another  class  of  moveable  bridges, 
used  for  the  same  purpose,  which  consist  of  a  platform  supported 
by  a  boat,  or  other  buoyant  body,  which  can  be  placed  in  or 
withdrawn  from  the  water-way,  as  circumstances  may  require. 

625.  Local  circumstances  will,  in  all  cases,  determine  what 
description  of  moveable  bridge  will  be  best.  If  the  width  of  the 
water-way  is  not  over  24  feet,  a  single  bridge  may  be  used  ;  but 
for  greater  widths  the  bridge  must  consist  of  two  symmetrical 
parts. 

626.  Draw-bridges.  When  the  horizontal  axis  of  this  de- 
scription of  bridge  is  placed  at  the  extremity  of  the  platform,  the 
bridge  is  manoeuvred  by  attaching  a  chain  to  the  other  extremity, 
which  is  connected  with  a  counterpoise  and  a  suitable  mechanism, 
by  which  the  slight  additional  power  required  for  raising  the 
bridge  can  be  applied. 

A  number  of  ingenious  contrivances  have  been  put  in  practice 
for  these  purposes.  They  consist  usually  either  of  a  counter- 
poise of  invariable  weight,  connected  with  additional  animal  mo 


MOVEABLE  BRIDGES. 


265 


tive  power,  which  acts  with  constant  intensity  but  with  a  variable 
arm  of  lever ;  or  of  a  counterpoise  of  variable  weight,  which  is 
assisted  by  animal  motive  power  acting  with  an  invariable  arm  of 
lever.  In  some  cases  the  bridge  is  worked  with  a  less  compli- 
cated combination,  by  dispensing  with  a  counterpoise,  and  ap- 
plying animal  motive  power,  of  variable  intensity,  acting  with  a 
constant  or  a  variable  arm  of  lever. 

Among  the   combinations  of  the  first  kind,  the  most  simple 
consists  in  placing  a  framed  lever  (Fig.  143)  revolving  on  a  hori 


Fig.  143— Shows  the  man- 
ner of  manoeuvring  a  draw- 
bridge either  by  a  framed 
lever,  or  by  a  counterpoise 
suspended  from  a  spiral 
eccentric. 

A,  abutment. 

a,  section  of  the  platform 

b,  framed  lever. 

c,  chain  attached  t)  the 
ends  of  the  lever  and  the 
platform. 

a,  strut  moveable  around 
its  lower  end. 

e,  bar  with  an  articulation 
at  each  end  that  confines 
the  strut  to  the  platform. 

/,  spiral  eccentric  connect- 
ed with  the  counterpoise 
g  by  a  chain  passing  over 
the  gorge  of  the  eccentric. 

h,  chain  for  raising  the 
bridge,  one  end  of  which 
is  attached  to  the  extre- 
mity of  the  platform,  and 
the  other  to  the  axle  of 
the  eccentric. 

i,  fixed  pulley  over  which 
the  chain  h  is  passed. 

m,  Wheel  fixed  to  the  axle 
of  the  eccentric  for  the 
purpose  of  turning  it  by 
means  of  animal  power 
applied  to  the  endless 
chain  n. 


zontal  axis  above  the  platform.  The  anterior  part  of  the  frame 
is  connected  with  the  moveable  extremity  of  the  platform  by  two 
chains.  The  posterior  portion,  which  forms  the  counterpoise, 
has  chains  attached  to  it  by  which  the  lever  can  be  worked  by 
men. 

When  the  locality  does  not  admit  of  this  arrangement,  the 
chain  attached  to  the  moveable  end  of  the  platform  may  be  con 
nected  with  a  horizontal  axle  above  the  platform,  to  which  is  also 
attached  a  fixed  eccentric  of  a  spiral  shape,  (Fig.  143,)  connected 
with  a  chain  that  passes  over  its  gorge  and  sustains  a  counter- 
poise of  invariable  weight.  Upon  the  same  axle  an  ordinary 
wheel  is  hung,  over  the  gorge  of  which  passes  an  endless  chain 
t(  manoeuvre  the  bridge  by  animal  power. 

Of  the  combinations  of  variable  counterpoises  the  mechanism 

34 


266 


BRIDGES,  ETC. 


of  M.  Poncelet,  which  has  been  successfully  applied  in  manv 
instances  in  France  for  the  draw-bridges  of  military  works,  is 
one  of  the  most  simple  in  its  arrangement  and  construction.  The 
moveable  end  of  the  platform  (Fig.  144)  is  connected  by  a  com- 

Fig.  144-Shows  the  ar- 
rangement of  a  draw- 
bridge with  a  varia- 
ble counter]  wise. 

A  and  B,  abutments. 

g,  variable  counter- 
poise formed  of  a 
chain  with  Hat  links, 
one  end  of  which  ia 
attached  to  a  fixed 
point,  and  the  other 
to  the  chain  e  attach- 
ed to  the  moveable 
end  of  the  platform. 

i,  fixed  pulley  over 
which  the  chain  c 
passes  to  the  smah 
wheel  k  fixed  on  a 
horizontal  shall,  tc 
which  is  also  attach- 
ed the  wheel  m  and 
the  endless  chain  n 
for  manoeuvring  the 
bridge. 

mon  chain,  that  passes  over  the  gorge  of  a  wheel  hung  upon  <- 
horizontal  shaft  above  the  platform,  with  another  chain  of  variable 
breadth,  formed  of  flat  bar  links,  which  forms  the  counterpoise. 
The  chain  counterpoise  is  attached  at  its  other  extremity  to  a 
fixed  point  in  such  a  way,  that  when  the  platform  ascends,  a  por- 
tion of  the  weight  of  the  chain  is  borne  by  this  fixed  point ;  and 
thus  the  weight  of  the  counterpoise  decreases  as  the  platform 
rises.  The  system  is  manoeuvred  by  an  endless  chain  passed 
over  the  gorge  of  a  wheel  hung  upon  the  horizontal  shaft. 

For  light  platforms  a  counterpoise  maybe  dispensed  with,  and 
the  bridge  may  be  manoeuvred  by  connecting  the  chain  attached 
to  the  moveable  end  of  the  platform  to  a  horizontal  shaft,  which 
is  turned  by  the  usual  tooth-work  combinations. 

When  the  locality  does  not  admit  of  manoeuvring  the  bridge  by 


p$S\ 


Fig.  1-45 — Shows  the  ar- 
rangement of  a  draw- 
bridge where  the  coun- 
terpoise is  formed  by 
prolonging  back  the 
platform. 

A,  abutment. 

B,  well  of  a  suitable 
torm  for  manoeuvring 
the  bridge. 

a,  chain-stay  to  keep 
the  platform  firm  when 
the  bridge  is  down. 


a  chain  connected  with  some  point  above  the  frame-work  of  the 
platform,  Fig.  145  is  continued  back,  from  two  thirds  to  three 


MOVEABLE  BRIDGES.  267 

fifths  its  length,  from  the  face  of  the  abutment,  to  form  a  coun- 
terpoise for  the  platform  of  the  bridge.  The  horizontal  axis  of 
the  bridge  is  placed  near  the  face  of  the  abutment,  and  a  well  of  a 
suitable  shape  to  receive  the  posterior  portion  of  the  platform  that 
forms  the  counterpoise  is  formed  behind  the  abutment. 

The  mechanism  for  working  the  bridge  may  consist  of  i  chain 
and  capstan  below  the  platform-counterpoise,  or  of  a  suitable 
combination  of  tooth-work. 

In  bridges  of  a  single  platform,  the  moveable  extremity,  wlie.:- 
the  bridge  is  lowered,  rests  on  the  opposite  abutment,  and  no 
intermediate  support  will  be  required  for  the  structure  if  the 
frame-work  be  of  sufficient  strength  ;  but  when  a  double  bridge, 
consisting  of  two  plat  forms,  is  used,  the  platforms  (Fig.  143) 
should  be  supported  near  their  moveable  ends,  when  the  bridge 
is  down,  by  struts  moveable  around  the  joint  by  which  they  are 
connected  with  the  face  of  the  abutments.  These  struts  are 
so  connected  with  the  bridge  that  they  are  detached  from  it 
and  drawn  up  when  it  is  raised,  and  fall  back  into  their  places, 
abutting  against  blocks  near  the  moveable  end  of  the  platform, 
when  the  bridge  is  down.  By  these  arrangements  the  chains  for 
working  the  bridge  are  relieved  from  a  portion  of  the  strain  when 
the  bridge  is  down,  and  it  is  also  rendered  more  firm. 

When  the  counterpoise  is  formed  by  the  rear  part  of  the  plat 
form,  additional  security  may  be  given  to  the  bridge  when  down 
by  attaching  two  chains  beneath  the  platform,  and  securing  them 
to  anchoring-points  at  the  bottom  of  the  well.  In  some  cases  a 
heavy  bar,  fitted  to  staples  beneath  connected  with  the  timbers 
of  the  platform,  is  used  for  the  same  purpose. 

In  double  bridges  the  two  platforms  when  lowered  should  abut 
against  each  other,  giving  a  slight  elevation  to  the  centre  of  the 
bridge.  This  not  only  gives  greater  stiffness,  but  is  favorable  to 
detaching  the  platforms  when  the  bridge  is  to  be  raised. 

For  draw,  and  every  kind  of  moveable  bridge,  temporary  bar- 
riers should  be  erected  on  each  side  at  the  entrance  upon  the 
bridge,  to  prevent  accidents  by  persons  attempting  to  cross  the 
bridge  before  it  is  properly  secured  when  lowered. 

627.  TvrTwng-briages,  These  bridges  revolve  horizontally 
upon  a  vertical  shaft,  or  gudgeon  below  the  platform,  which  is 
usually  thrown  far  enough  back  from  the  face  of  the  abutment  to 
place  the  side  of  the  bridge,  when  brought  round,  just  within  this 
face.  The  weights  of  the  parts  of  the  bridge  around  the  shaft 
should  balance  each  other. 

To  support  and  manoeuvre  the  bridge  (Fig.  146)  a  circular 
ring  of  iron,  or  roller-way,  of  less  diameter  than  the  breadth  of 
.he  bridge,  and  concentric  with  the  venical  shaft,  is  firmly  im- 
oedded  in  masonry.     Fixed  rollers,  in  the  shaoe  of  truncated 


268 


BRIDGES,  ETC. 


cones,  are  attached  at  equal  distances  apart  to  the  frame*work  of 
the  platform  beneath,  and  rest  upon  the  roller-way-     The  bridge 


Fig.  146— Represents  the  arrangement  of  a  turning-bridge. 

a,  platform  ot  the  bridge. 

b,  vertical  posts  to  which  the  iron  stays  n,  n  are  attached. 

c,  vertical  shaft  or  gudgeon  on  which  the  bridge  turns, 
o,  o,  conical  rollers. 

is  worked  by  a  suitably  arranged  tooth-work,  or  by  a  chain  ana 
capstan.  In  some  cases  cast-iron  balls,  resting  on  a  grooved 
roller-way  and  fitting  into  one  of  corresponding  shape  fixed  be- 
neath the  platform,  have  been  used  for  manoeuvring  the  bridge. 

The  ends  of  the  bridge  are  cut  in  the  shape  of  circular  arcs  tc 
fit  recesses  of  a  corresponding  form  in  the  abutments,  so  arranged 
as  not  to  impede  the  play  of  the  bridge. 

In  double  turning-bridges  the  two  ends  of  the  platforms  which 
come  together  should  be  of  a  curved  shape.  The  platforms 
should  be  sustained  from  beneath  by  struts,  like  those  used  for 
draw-bridges,  which  can  be  detached  and  drawn  into  recesses 
when  the  passage  is  interrupted  ;  or  else  they  may  be  arranged 
with  a  ball-and-socket  joint  at  their  lower  extremity,  so  as  to  be 
brought  round  with  the  bridge.  For  the  purpose  of  giving  addi- 
tional strength  and  security  to  the  bridge,  iron  stays  are,  in  some 
cases,  attached  on  each  side  of  the  platform  near  the  extremities, 
and  connected  with  vertical  posts  placed  in  a  line  with  the  verti 
cal  shaft. 

Turning-bridges  may  be  made  either  of  timber,  or  of  cast  iron  ; 
the  latter  material  is  the  more  suitable,  as  admitting  of  more  ac- 
curacy of  workmanship,  and  not  being  liable  to  the  derangements 
caused  by  the  shrinking  or  warping  of  frame-work  of  timber. 

628.  Rolling-bridges.  These  bridges  are  placed  upon  fixed 
rollers,  so  thai  they  can  be  moved  forward  or  backward,  to  inter- 
rupt, or  open  the  communication  across  the  water-way.  The 
part  of  the  bridge  that  rests  upon  the  rollers,  when  the  passage 
is  closed,  must  form  a  counterpoise  to  ihe  other.  The  mechan- 
ism usually  employed  for  manoeuvring  these  bridges  consists  of 
tooth-work,  and  may  be  so  arranged  that  it  can  be  worked  by 
one  or  more  persons  standing  on  the  bridge.  Instead  of  fixed 
rollers  turning  on  axl  :-s,  iron  balls  resting  in  a  grooved  roller-way 


AQUEDUCT-BRIDGES.  269 

may  be  ned,  a  similar  roller-way  being  affixed  to  the  ft .  me-work 
beneath. 

629.  Boat-bridge.  A  moveable  bridge  of  this  kind  may  be 
made  by  placing  a  platform  to  form  a  roadway  upon  a  boat,  or  a 
water-tight  box  of  a  suitable  shape.  This  bridge  is  placed  in,  or 
withdrawn  from  the  water-way,  as  ch-cumstances  may  require,  a 
suitable  recess  or  mooring  being  arranged  for  it  near  the  water 
way  when  it  is  left  open. 

A  bridge  of  this  character  cannot  be  conveniently  used  in  tidal 
waters,  except  at  certain  stages  of  the  water.     It  may  be  em 
ployed  with  advantage  on  canals  in  positions  where  a  fixed  bridge 
could  not  be  placed. 

AQUEDUCT-BRIDGES. 

630.  In  aqueducts  and  aqueduct-bridges  of  masonry,  for  sup- 
plying reservoirs  for  the  wants  of  a  city,  or  for  any  other  purpose, 
the  volume  of  water  conveyed  being,  generally  speaking,  small, 
the  structure  will  present  no  peculiar  difficulties  beyond  affording 
a  water-tight  channel.  This  may  be  made  either  of  masonry,  01 
of  cast-iron  pipes,  according  to  the  quantity  of  water  to  be  deliv- 
ered. If  formed  of  masonry,  the  sides  and  bottom  of  the  channel 
should  be  laid  in  the  most  careful  manner  with  hydraulic  cement, 
and  the  surface  in  contact  with  the  water  should  receive  a  coating 
of  the  same  material,  particularly  if  the  stone  or  brick  used  be 
of  a  porous  nature.  This  part  of  the  structure  should  not  be 
commenced  until  the  arches  have  been  uncentred  and  the  heavier 
parts  of  the  structure  have  been  carried  up  and  have  had  time  to 
settle.  The  interior  spandrel-filling,  to  the  level  of  the  masonry 
which  forms  the  bottom  of  the  water-way,  may  either  be  formed 
of  solid  material,  of  good  rubble  laid  in  hydraulic  cement,  or  of 
beton  well  settled  in  layers;  or  a  system  of  interior  walls,  like 
those  used  in  common  bridges  for  the  support  of  the  roadway, 
may  be  used  in  this  case  for  the  masonry  of  the  water-way  to 
rest  on. 

631.  In  canal  aqueduct-bridges  of  masonry,  as  the  volume  of 
water  required  for  the  purposes  of  navigation  is  much  greater 
than  in  the  case  of  ordinary  aqueducts,  and  as  the  structure  has 
to  be  traversed  by  horses,  every  precaution  should  be  taken  to 
procure  great  solidity,  and  secure  the  work  from  accidents. 

jincnt  arches  of  medium  span  will  generally  be  found  most 
suitable  for  works  of  this  character.  The  section  of  the  water- 
way is  generally  of  a  trapezoidal  form,  the  bottom  line  being 
horizontal,  and  the  two  sides  receiving  a  slight  batir ;  its  dimen- 
sions are  usually  restricted  to  allow  the  passage  of  a  single  boat 
at  a  time.     On  one  side  of  the  water-way  a  horse  or  tow  path  if 


270  BRIDGES,  Elu. 

placed,  and  on  the  other  a  narrow  footpath.  The  water-way 
should  be  faced  with  a  hard  cut-stone  masonry,  well  bonded  to 
secure  it  from  damage  from  the  passage  of  the  boats.  The  space 
between  the  facing  of  the  water-way,  termed  the  trunk  of  the 
aqueduct,  and  the  head-walls,  is  filled  21  with  solid  material,  either 
of  rubble  or  of  beton. 

A  parapet-wall  of  the  ordinary  form  and  dimensions  surmounts 
the  tow  and  footpaths. 

The  approach  to  an  aqueduct-bridge  from  a  canal  is  made  by 
gradually  increasing  the  width  of  the  trunk  between  the  wings, 
which,  for  this  purpose,  usually  receives  a  curved  shape,  and 
narrowing  the  water-way  of  the  canal  so  as  to  form  a  convenient 
access  to  the  aqueduct.  Great  care  should  be  taken  to  form  a 
perfectly  water-tight  junction  between  the  two  works. 

632.  When  cast  iron  or  timber  is  used  for  the  trunk  of  an 
aqueduct-bridge,  the  abutments  and  piers  should  be  built  of  stone. 
The  trunk,  which,  if  of  cast  iron,  is  formed  of  plates  with  flanchcs 
to  connect  them,  or,  if  of  timber,  consists  of  one  or  two  thick- 
nesses of  plank  supported  on  the  outside  by  a  framing  of  scant- 
ling, may  be  supported  by  a  bridge-frame  of  cast  iron,  or  of  tim- 
ber, or  be  suspended  from  chains  or  wire  cables. 

The  tow-path  may  be  placed  either  within  the  water-way,  or, 
as  is  most  usually  done,  without.  It  generally  consists  of  a  sim- 
ple flooring  of  plank  laid  on  cross-joists  supported  from  beneath 
by  suitably  arranged  frame-work. 

633.  The  following  succinct  descriptions  of  some  of  the  aque- 
duct-bridges of  the  United  States  and  of  Europe  are  derived  from 
authentic  sources. 

Chirk  Aqueduct-bridge  over  the  Ceriog.  This  work,  built  by 
Telford,  consists  of  10  full  centre  arches  of  masonry,  of  40  feet 
span  each.  The  water-way  is  only  1 1  feet  wide  and  5  feet  deep. 
The  tow-path  6  feet  wide. 

The  piers  of  this  work,  which  in  some  places  are  over  100  feet 

i'   height,  are  built  hollow  for  some  distance  below  the  top  ;  the 

icing  being  connected  by  cross-walls  upon  which  the  bottom 

jf  the  water-way,  formed  of  broad  iron-flanched  plates,  and  the 

masonry  of  the  sides  rest. 

Pont-y-Cy  stile  Aqueduct-bridge  over  the  Dee.  This  is  also 
one  of  Telford's  early  works.  The  trunk  is  of  cast-iron  plates 
connected  by  flanches.  These  rest  upon  stone  piers  and  upon  a 
0  ridge-frame  of  cast  iron  consisting  of  four  ribs  of  solid  panels 
The  span  of  the  ribs  is  45  feet  and  the  rise  t\  feet. 

The  breadth  of  the  water-way  is  11  feet  10  inches.  The  tow- 
path  is  4  feet  8  inches  wide,  and  is  placed  within  the  water-way, 
resting  upon  cast-iron  uprights. 

The  canal  aqueduct-bridges  at  Guttin  over  the  Allier,  and  at 


AQUEDUCT-BRIDGES.  271 

Digotn  upon  the  Loire,  are  among  the  more  recent  structures  of 
this  character  in  France.  They  are  both  built  upon  the  same 
plan,  and  of  mixed  masonry.  The  first  has  eighteen  arches ; 
the  second  eleven.  The  span  of  each  arch  is  52|  feet,  and  the 
rise  about  23  feet.  The  piers  are  about  1 0  feet  thick  at  the  im- 
post. The  breadth  of  the  aqueduct  between  the  heads  is  31  feet, 
and  that  of  the  water-way  about  16  feet. 

Rochester  Canal  Aqueduct-bridge.  This  is  the  most  recent 
and  the  largest  aqueduct-bridge  built  entirely  of  masonry  in  the 
United  States.  It  consists  of  seven  segment  arches.  Its  water 
way  is  of  sufficient  width  for  the  passage  of  two  boats,  and  is 
adapted  to  the  enlargement  of  the  Erie  canal.  The  span  of  each 
arch  is  52  feet;  the  rise  10  feet.  The  key-stone  is  2  feet 
6  inches  in  depth,  and  the  top  of  it  is  on  a  level  with  the  bottom 
of  the  trunk.  The  piers  are  10  feet  thick  at  the  impost.  The 
water-way  is  9  feet  in  depth,  the  masonry  of  the  sides  receiving 
a  batir  of  2  inches  in  one  foot.  The  depth  of  water  is  7  feet, 
and  the  width  at  the  water-line  45  feet.  The  sides  of  the  water- 
way, the  top  surface  of  which  forms  the  tow-paths,  are  1 1  feet  in 
width  at  top,  including  the  projection  of  the  coping.  The  trunk 
at  each  extremity  is  gradually  enlarged,  in  a  curved  shape,  to  the 
width  of  55  feet,  where  it  unites  with  the  slopes  of  the  water-way 
of  the  canal. 

This  work  is  built  throughout  in  a  very  strong  and  superior 
manner,  of  heavy  blocks  of  gray  lime-stone  laid  in  hydraulic 
mortar. 

Potomac  Canal  Aqueduct-bridge.  This  work,  originally  in- 
tended to  be  of  stone  throughout,  was  to  have  consisted  of  twelve 
oval  arches  of  eleven  centres,  the  span  of  each  being  100  feet, 
and  the  rise  25  feet.  Every  third  pier  forms  an  abutment-pier, 
and  is  21  feet  thick  at  the  impost ;  the  others  are  only  12  feet 
thick  at  the  same  level.  The  piers  have  been  built  upon  the 
original  design,  but  a  wooden  superstructure,  consisting  of  the 
trunk  of  the  aqueduct,  a  tow-path,  and  the  frame-work  for  their 
support,  has  been  substituted  for  the  stone  arches. 

The  trunk  (Fig.  147)  is  formed  of  a  frame  consisting  of  two 
parallel  open-built  beams,  connected  at  bottom  by  parallel  cross- 
joists  and  horizontal  diagonal  braces,  which  are  sheathed  on  the 
interior  with  plank  to  form  the  water-way. 

Each  of  the  open-built  beams  is  composed  of  a  top  and  bottom 
string,  connected  by  uprights  that  project  above  and  below  the 
strings,  and  by  single  cliagor  al  braces  placed  between  each  pair 
of  uprights. 

The  tow-path  is  placed  on  the  outside  of  the  trunk,  and  con 
sists  of  a  flooring  laid  upon  cross-jcists  placed  between  one  of  the 
built  beams  of  the  trunk  and  a  thut1  parallel  to  it. 


272 


BRIDGES,  ETC. 


The  exterior-built  beam  of  the  tow-path  is  framed  of  smaller 
scantling  than  the  other  two.     It  is  connected  with  the  built 


Fig.  147 — Represents  a  cross  section  of  the  trunk  and  tow-path  of  the 
Potomac  canal  aqueduct-bridge. 

A,  interior  of  trunk. 

B,  tow-path. 

a,  a,  uprights  of  the  open-built  beams  on  the  sides  of  the  trunk. 

b,  upright  of  the  open-built  beam  of  the  tow-path. 

c,  lower  strings  of  the  built  beams. 
a,  upper  string. 

e,  cross-joists  on  which  the  sheathing  of  the  bottom  of  the  trunk  rests. 

n,  eross-joists  of  the  tow-path. 

m,  vertical  diagonal  braces  between  the  cross  joists. 

/,  parapet. 

beam  of  the  trunk  by  every  fourth  cross-joist  of  the  trunk,  by  the 
top  cross-joists  of  the  flooring,  and  by  vertical  diagonal  braces 
placed  between  each  pair  of  top  and  bottom  cross-joists. 

The  uprights  of  the  exterior-built  beam  of  the  tow-path  pro- 
ject sufficiently  high  above  the  flooring  to  form  a  parapet. 

The  frame-work  of  the  trunk  and  tow-path  is  supported  at 
intermediate  points  from  beneath  by  inclined  struts  which  abut 
against  the  faces  of  the  piers  at  a  point  above  the  high-water 
level. 

The  section  of  the  water-way  is  rectangular.  The  interior 
width  is  17  feet;  the  height  of  the  sheathing  8  feet  4  inches 
within  ;  and  the  depth  of  water  4  feet  4  inches. 

The  surface  of  the  tow-path  is  6  feet  wide  between  the  uprights 
of  the  built  beams,  and  is  on  a  level  with  the  top  of  the  sheathing. 
The  exterior  parapet  is  3  feet  10  inches  above  the  level  of  the 
tow-path,  and  an  interior  parapet,  2  feet  above  the  same  level,  is 
formed  by  a  capping  on  the  uprights  of  the  built  beam,  making 
the  height  of  the  capping  on  each  side  of  the  trunk  10  feet  4 
inches  above  the  sheathing  of  the  bottom. 

The  frame-work  of  this  structure  is  simple  in  its  combinations 
and  well  arranged  both  for  strength  and  stiffness. 

Wire  Suspension  Canal  Aqueduct-bridge  over  the  Alleghany 
river  at  Pittsburgh.     This  novel  work  (Fig.  148)  was  planned 


AQUEDUCT-BRIDGES. 


273 


Fig.  148— shows  in  elevation  a  portion  of  the  stone  supports,  and  a  croak 
section  of  the  trunk,  &c,  of  the  Alleghany  canal  aqueduct-bridge. 

A,  piers. 

B,  Supports  of  masonry  on  the  piers  for  the  wire  cables. 

C,  interior  of  a  portion  of  the  trunk. 

a,  cross-joists  suspended  from  the  cables  m  by  the  bent  6uspt,nding-bars  r«, 
on  which  the  bottom  e  of  trunk  rests. 

b,  inclined  struts  in  pairs  connected  with  the  pieces  c  to  support  the  Bides  d 

of  the  trunk. 

D,  tow-path. 

«,  cross-joists  of  the  tow-path. 

r,  inclined  supports  of  «. 

t  and  v,  parapets. 

A,  sleepers  on  top  of  the  piers  on  which  the  cross-joists  a  rest. 

and  constructed  by  Mr.  Roebling,  through  whom  the  following 
detailed  description  was  obtained  : 

"  This  work  is  formed  of  seven  spans  of  160  feet  each  from 
centre  to  centre  of  pier.  The  trunk  is  of  wood  and  1140  feet 
long,  14  feet  wide  at  bottom,  16^  feet  wide  on  top ;  the  sides  85 
feet  deep.  These  as  well  as  the  bottom  are  composed  of  a 
double  course  of  2\  inch  white-pine  plank  laid  diagonally,  the 
two  courses  crossing  each  other  at  right  angles,  so  as  to  form  a 
solid  lattice-work  of  great  strength  and  stiffness,  sufficient  to  bear 
its  own  weight  and  resist  the  effects  of  the  most  violent  storms. 
The  bottom  of  the  trunk  rests  upon  transverse  beams,  arranged 
in  pairs  4  feet  apart ;  between  these  the  posts  which  support  the 
sides  of  the  trunk  are  let  i»  with  dove-tailed  tenons,  secured  by 
bolts.  The  outside  posts  which  support  the  side-walk  and  tow- 
path  incline  outwards  and  are  connected  with  the  beams  in  a 
similar  Planner.  Each  trunk-post  is  held  by  two  braces  2^x10 
inches,  and  connected  with  the  outside  posts  by  a  double  joist  of 
2^x10.     The  trunk-posts  are  7  inches  square  at  the  top  and 

35 


274 


BRIDGES,  ETC. 


7x14  at  the  heel.  The  transverse  beams  are  27  feet  long  16 
inches  deep,  and  6  inches  wide  ;  the  space  between  the  two  ad- 
joining is  4  inches.  It  will  be  observed  that  all  parts  of  the 
frame,  with  the  exception  of  the  posts,  are  double,  so  as  to  admit 
the  suspension-rods.  Each  pair  of  beams  is  supported  on  each 
side  of  the  trunk  by  double  suspending-rods  of  1{  inch  round 
bar-iron,  bent  in  the  shape  of  a  stirrup,  and  mounted  on  a  small 
cast-iron  saddle,  which  rests  on  the  cable.  These  saddles  are  on 
top  of  the  cables  connected  by  links,  which  diminish  in  size  from 
the  pier  towards  the  centre.  The  sides  of  the  trunk  rest  solid 
against  the  bodies  of  masonry,  which  are  erected  on  each  pier 
and  abutment  as  bases  for  the  pyramids  which  support  the  cables. 
These  pyramids,  which  are  constructed  of  three  blocks  or  courses 
of  a  durable  coarse-grained  hard  mountain  sand-stone,  rise  o  feet 
above  the  level  of  the  side-walk  and  tow-path,  and  measure  3x5 
feet  on  top,  and  4  x6£  feet  in  base.  The  side-walk  and  tow-path 
being  7  feet  wide,  leave  3  feet  space  outside  for  the  passage  of 
he  pyramids  ;  the  ample  width  of  the  tow  and  footpath  is  there- 
fore contracted  on  every  pier ;  but  this  arrangement  proves  no 
inconvenience,  and  was  necessary  for  the  suspension  of  the  cables 
next  to  the  trunk. 

"  As  the  caps  which  cover  the  saddles  and  cables  on  the  pyra- 
mids rise  3  feet  above  the  inside,  or  trunk-railing,  they  would 
obstruct  the  passage  of  the  tow-line ;  this  however  is  obviated 
by  a  slide-rod  of  round  iron,  which  passes  over  the  top  of  the  cap 
and  forms  a  gradual  slope  down  to  the  railing  on  each  side  of  the 
pyramid. 

"  The  wire  cables,  which  are  the  main  support  of  the  structure, 
are  suspended  next  to  the  trunk,  one  on  each  side.  Each  of 
these  two  cables  is  exactly  7  inches  in  diameter,  perfectly  solid 
and  compact,  and  constructed  in  one  piece  from  shore  to  shore, 
1175  feet  long  ;  it  is  composed  of  1900  wires  of  |  inch  diameter, 
which  are  laid  parallel  to  each  other.  Great  care  has  been  taken 
to  insure  an  equal  tension  of  the  wires.  The  oxidation  of  the 
wires  is  guarded  against  by  a  varnish  applied  to  each  separately. 
The  preservation  of  the  cables  is  insured  by  a  close,  compact, 
and  continuous  wrapping,  made  of  annealed  wire  and  laid  on  by 
machinery  in  the  most  perfect  maimer 

"  The  extremities  of  the  cables  on  the  aqueduct  do  not  extend 
below  ground,  but  connect  with  anchor-chains,  which  in  a  curved 
line  pass  through  large  masses  of  masonry,  the  last  links  occupy- 
ing a  vertical  position.  The  bars  composing  these  chains  aver- 
age 1^x4  inches,  and  are  from  4  to  12  feet  long;  they  are 
manufactured  of  boiler-scrap,  and  forged  in  one  piece  will  out  a 
Weld.  The  extreme  links  are  anchored  to  heavy  cast-iron  plates 
rf  6  feet  square,  which  are  held  down  by  the  foundations,  upor 


AQUrDUCT-BRIDGES.  275 

which  tbe  weight  of  700  perches  of  masonry  rests.  The  stability 
of  this  r>a:i  of  the  structure  is  fully  insured,  as  the  resistance  of 
the  anchorage  is  twice  as  great  as  the  greatest  strain  to  which  the 
chains  can  ever  be  subjected. 

"  The  plan  of  anchorage  adopted  on  the  aqueduct  varies  mate- 
rially from  those  methods  usually  applied  to  suspension  bridges, 
where  an  open  channel  is  formed  under  ground  for  the  passage 
of  the  chains.  The  chains  below  ground  are  imbedded  and  com- 
pletely surrounded  by  cement.  In  the  construction  of  the  ma- 
sonry this  material  and  common  lime-mortar  have  been  abundantly 
applied.  The  bars  are  painted  with  red  lead  :  their  preservation 
is  rendered  certain  by  the  known  quality  of  calcareous  cements  tc 
prevent  oxidation.  If  moisture  should  find  its  way  to  the  chains, 
it  will.be  saturated  with  lime,  and  add  another  calcareous  coat- 
ing to  the  iron.  This  portion  of  the  work  has  been  executed 
with  scrupulous  care,  so  as  to  render  it  unnecessary,  on  the  part 
of  those  who  exercise  a  surveillance  over  the  structure,  to  examine 
it.  The  repainting  of  the  cables  every  two  or  three  years  will 
insure  their  duration  for  a  long  period. 

"  Where  the  cables  rest  on  the  saddles,  their  size  is  increased 
at  two  points,  by  introducing  short  wires  and  forming  swells 
which  fit  into  corresponding  recesses  of  the  casting.  Between 
these  swells  the  cable  is  forcibly  held  down  by  three  sets  of 
strong  iron  wedges,  driven  through  openings  which  are  cast  in 
the  sides  of  the  saddle.  During  the  raising  of  the  frame-work, 
the  several  arches  were  frequently  subjected  to  very  unequal  and 
considerable  forces,  which  never  disturbed  the  balance,  and  proved 
the  correctness  of  previous  calculations.  The  woodwork  in  any 
of  the  arches,  separately,  may  be  removed  and  substituted  by  new 
material,  without  affecting  the  equilibrium  of  the  next  one. 

"  The  original  idea  upon  which  the  plan  has  been  perfected, 
was  to  form  a  wooden  trunk,  strong  enough  to  support,  its  own 
weight,  and  stiff  enough  for  an  aqueduct,  or  bridge,  and  to  com- 
bine this  structure  with  wire  cables,  of  a  sufficient  strength  tc 
bear  safely  the  great  weight  of  water. 

"  Table  of  Quantities  on  Aqueduct. 

Length  of  aqueduct  without  extensions 
Length  of  cables      ...... 

Length  of  cables  and  chains     .... 

Diameter  of  cables  ..... 

Aggregate  weight  of  both  cables       .  •       . 
Section  of  4  feet  of  water  in  trunk    . 
Total  weight  of  water  in  aqueduct    . 
Do.  do.  in  one  span    . 

Weight  of  one  span  including  all 
Aggregate  number  of  wires  in  both  cables 


1140  feet. 

1175    " 

1283    " 

7  inches. 

110  tons. 

59  superf. 

feel 

2100  tons. 

295    " 

- 

420    " 

3800 

276 


BRIDGES,  ETC. 


Aggregate  solid  sectior  of  both  cables 
Do.  do.  anchor-chains 

Deflection  of  cables  .... 

Elevation  from  top  of  pyramids  to  top  of  piers 

Weight  of  water  in  one  span  between  piers 

Tension  of  cables  resulting  from  this  weight 

Tension  of  one  single  wire 

Average  ultimate  strength  of  one  wire 

Ultimate  strength  of  cables 

Tension  resulting  from  weight  of  water  upon  1  solid 
square  inch  of  wire  cable 

Tension  resulting  from  weight  of  water  upon  1  square 
inch  of  anchor-chains 

Pressure  resulting  from  water  upon  a  pyramid 
Do.  upon  one  superficial  foot 


53  superf. 

inch 

72           * 

14  feet  6  incV 

16    "     6 

u 

275  tons. 

392    " 

206  lbs. 

1100  " 

2090  tons. 

14800  lbs. 

11000   " 

137i  ton> 

18400"lbs.'» 

See  Vote  A.,  Appendix. 


ROADS.  27" 


ROADS. 


634.  In  establishing  a  line  of  internal  communication  of  any 
character,  whether  it  be  an  ordinary  road,  railroad,  or  canal,  the 
main  considerations  to  which  the  attention  of  the  engineer  must 
be  directed  in  the  outset  are — 1,  the  probable  character  and 
amount  of  traffic  over  the  line  ;  2,  the  wants  of  the  community 
in  the  neighborhood  of  the  line ;  3,  the  natural  features  of  the 
country,  between  the  points  of  arrival  and  departure,  as  regards 
their  adaptation  to  the  proposed  communication. 

As  the  last  point  alone  comes  exclusively  within  the  province 
of  the  engineer's  art,  and  within  the  limits  prescribed  to  this  work, 
attention  will  be  confined  solely  to  its  consideration. 

635.  Reconnaissance.  A  thorough  examination  and  study  of 
the  ground  by  the  eye,  termed  a  reconnaissance,  is  an  indis- 
pensable preliminary  to  any  more  accurate  and  minute  survey 
by  instruments,  to  avoid  loss  of  time,  as  by  this  more  rapid  ope- 
ration any  ground  unsuitable  for  the  proposed  line  will  be  as  cer- 
tainly detected  by  a  person  of  some  experience,  as  it  could  be  by 
the  slow  process  of  an  instrumental  survey.  Before  however  pro- 
ceeding to  make  a  reconnaissance,  a  careful  inspection  of  the 
general  maps  of  that  portion  of  the  country  through  which  the 
communication  is  to  pass,  will  facilitate,  and  may  considerably 
abridge,  the  labors  of  the  engineer ;  as  from  the  natural  features 
laid  down  upon  them,  particularly  the  direction  of  the  water- 
courses, he  will  at  once  be  able  to  detect  those  points  which  will 
be  favorable,  or  otherwise,  to  the  general  direction  selected  for 
the  line.  This  will  be  sufficiently  evident  when  it  is  considered 
—  1,  that  the  water-courses  are  necessarily  the  lowest  lines  of 
the  valleys  through  which  they  flow,  and  that  their  direction  must 
also  be  that  of  the  lines  of  greatest  declivity  of  their  respective 
valleys  ;  2,  that  from  the  position  of  the  water-courses  the  position 
also  of  the  high  grounds  by  which  they  are  separated  naturally 
follows,  as  well  as  the  approximate  position  at  least  of  the  ridges, 
or  highest  lines  of  the  high  grounds,  which  separate  their  opposite 
slopes,  and  which  are  at  the  same  time  the  lines  of  greatest  de- 
clivity common  to  these  slopes,  as  the  water-courses  are  the  cor 
responding  lines  of  the  slopes  that  form  the  valleys. 

Keeping  these  facts  (which  are  susceptible  of  rigid  mathemati 
cal  demonstration)  in  view,  it  will  be  practicable,  from  a  careful 
examination  of  an  ordinary  general  map,  if  accurately  constructed, 
not  only  to  trace,  with  considerajle  accuracy,  the  general  direc 


278  ROADS. 

tion  of  the  ridges  from  having  that  of  the  water-courses,  but  alsc 
to  detect  those  depressions  in  them  which  will  be  favorable  to  the 
passage  of  a  communication  intended  to  connect  two  main  or  two 
secondary  valleys.  The  following  illustrations  may  serve  to  place 
this  subject  in  a  clearer  aspect. 

If,  for  example,  it  be  found  that  on  any  portion  of  a  map  the 
water-courses  seem  to  diverge  from  or  converge  towards  one  point, 
it  will  be  evident  that  the  ground  in  the  first  case  must  be  the 
common  source  or  supply  of  the  water-courses,  and  therefore  the 
highest ;  and  in  the  second  case  that  it  is  the  lowest,  and  forms 
their  common  recipient. 

If  two  water-courses  flow  in  opposite  directions  from  a  common 
point,  it  will  show  that  this  is  the  point  from  which  they  derive 
their  common  supply,  at  the  head  of  their  respective  valleys,  and 
that  it  must  be  fed  by  the  slopes  of  high  grounds  above  this  point ; 
or,  in  other  words,  that  the  valleys  of  the  two  water-courses  are 
separated  by  a  chain  of  high  grounds,-  which,  at  the  point  where 
it  crosses  them,  presents  a  depression  in  its  ridge,  which  would 
be  the  natural  position  for  a  communication  connecting  the  two 
valleys. 

If  two  water-courses  flow  in  the  same  direction  and  parallel  to 
each  other,  it  will  simply  indicate  a  general  inclination  of  the 
ridge  between  them,  in  the  same  direction  as  that  of  the  water- 
courses. The  ridge,  however,  may  present  in  its  course  eleva- 
tions and  depressions,  which  will  be  indicated  by  the  points  in 
which  the  water-courses  of  the  secondary  valleys,  on  each  side 
of  it,  intersect  each  other  on  it ;  and  these  will  be  the  lowest 
points  at  which  lines  of  communication,  through  the  secondary 
valleys,  connecting  the  main  water-courses,  would  cross  the  divi- 
ding ridge. 

If  two  water-courses  flow  in  the  same  direction,  and  parallel 
to  each  other,  and  then  at  a  certain  point  assume  divergent  direc- 
tions, it  will  indicate  that  this  is  the  lowest  point  of  the  ridge  be- 
tweeen  them. 

If  two  water-courses  flow  in  parallel  but  opposite  directions, 
depressions  in  the  ridge  between  them  will  be  shown  by  the 
meeting  of  the  water-courses  of  the  secondary  valleys  on  the 
ridge  ;  or  by  an  approach  towards  each  other,  at  any  point,  of 
the  two  principal  water-courses. 

Furnished  with  the  data  obtained  from  the  maps,  the  charactei 
of  the  ground  should  be  carefully  studied  both  ways  by  the  en 
gineer,  first  from  the  point  of  departure  to  that  of  arrival,  and  then 
returning  from  the  latter  tc  the  former,  as  without  this  double 
traverse  natural  features  of  essential  importance  might  escape 
he  eye. 

636.  Surveys.     From  the  results  of  the  reconnaissance,  the 


ROADS.  279 

engineer  will  be  able  to  direct  understanumgly  the  requisite  sur 
veys,  which  consist  in  measuring  the  lengths,  determining  the 
directions,  and  ascertaining  both  the  longitudinal  and  cross  levels 
of  the  different  routes,  or,  as  they  are  termed,  trial  lines,  with 
sufficient  accuracy  to  enable  him  to  make  a  comparative  estimate 
both  of  their  practicability  and  cost.  As  the  expense  of  making 
the  requisite  surveys  is  usually  but  a  small  item  compared  with 
that  of  constructing  the  communication,  no  labor  should  be  spared 
in  running  every  practicable  line,  as  otherwise  natural  features 
might  be  overlooked  winch  might  have  an  important  influence  on 
the  cost  of  construction. 

637.  Map  and  Memoir.  The  results  of  the  surveys  are  ac- 
curately embodied  in  a  map  exhibiting  minutely  the  topographical 
features  and  sections  of  the  different  trial  lines,  and  in  a  memoir 
which  should  contain  a  particular  description  of  those  features  of 
the  ground  that  cannot  be  shown  on  a  map,  with  all  such  infor- 
mation on  other  points  that  may  be  regarded  as  favorable,  or 
otherwise,  to  the  proposed  communication  ;  as,  for  example,  the 
nature  of  the  soil,  that  of  the  water-courses  met  with,  &c,  &c. 

638.  Location  of  common  Roads.  In  selecting  among  the 
different  trial-lines  of  the  survey  the  one  most  suitable  to  a  com- 
mon road,  the  engineer  is  less  restricted,  from  the  nature  of  the 
conveyance  used,  than  in  any  other  kind  of  communication.  The 
main  points  to  which  his  attention  should  be  confined  are — 1,  to 
connect  the  points  of  arrival  and  departure  by  the  most  direct,  or 
shortest  line  ;  2,  to  avoid  unnecessary  ascents  and  descents,  or, 
in  other  words,  to  reduce  the  ascents  and  descents  to  the  smallest 
practicable  limit ;  3,  to  adopt  such  suitable  slopes,  or  gradients, 
for  the  axis,  or  centre  line  of  the  road,  as  the  nature  of  the  con- 
veyance may  demand  ;  4,  to  give  the  axis  such  a  position,  with  re- 
gard to  the  surface  of  the  ground  and  the  natural  obstacles  to  be 
overcome,  that  the  cost  of  construction  for  the  excavations  and 
embankments  required  by  the  gradients,  and  for  the  bridges  and 
other  accessories,  shall  be  reduced  to  the  lowest  amount. 

639.  Deviations  from  the  right  line  drawn  on  the  map,  between 
the  points  of  arrival  and  departure,  will  be  often  demanded  by  the 
natural  features  of  the  ground.  In  passing  the  dividing  ridges 
of  main,  or  secondary  valleys,  for  example,  it  will  frequently  be 
found  more  advantageous,  both  for  the  most  suitable  gradients, 
and  to  diminish  the  amount  of  excavation  and  embankment,  to 
cross  the  ridge  at  a  lower  point  than  the  one  in  which  it  is  inter- 
sected by  the  right  line,  deviating  from  the  right  line  either 
towards  the  head,  or  upper  pari  of  the  valley,  or  towards  its  out- 
let, according  to  the  advantages  presented  by  the  natural  features 
of  the  ground,  both  for  reducing  the  gradients  and  the  amount  oi 
excavation  and  embankment. 


2&U  ROADS. 

Where  the  right  line  intersects  either  a  marsh,  or  water-course, 
it  may  be  found  less  expensive  to  change  the  direction,  avoiding 
the  marsh,  or  intersecting  the  water-course  at  a  point  where  the 
cost  of  construction  of  a  bridge,  or  of  the  approaches  to  it,  wiT 
be  moie  favorable  than  the  one  in  which  it  is  intersected  by  the 
right  line. 

Changes  from  the  direction  of  the  right  line  may  also  be  fa- 
vorable tor  the  purpose  of  avoiding  the  intersection  of  secondary 
water-courses  ;  of  gaining  a  better  soil  for  the  roadway  ;  of  giv- 
ing a  better  exposure  of  its  surface  to  the  sun  and  wind ;  or  of 
procuring  better  materials  for  the  road-covering. 

By  a  careful  comparison  of  the  advantages  presented  by  these 
different  features,  ihe  engineer  will  be  enabled  to  decide  how  far 
the  general  direction  of  the  right  line  may  be  departed  from  with 
advantage  to  the  location.  By  choosing  a  more  sinuous  course  the 
length  of  the  line  will  often  not  be  increased  to  any  very  consider 
able  degree,  while  the  cost  of  construction  may  be  greatly  re- 
duced, either  in  obtaining  more  favorable  gradients,  or  in  lessening 
the  amount  of  excavation  and  embankment. 

640.  When  the  points  of  arrival  and  departure  are  upon  dif- 
ferent levels,  as  is  usually  the  case,  it  will  seldom  be  practicable 
to  connect  them  by  a  continual  ascent.  The  most  that  can  be 
done  will  be  to  cross  the  dividing  ridges  at  their  lowest  points, 
and  to  avoid,  as  far  as  practicable,  the  intersection  of  considerable 
secondary  valleys  which  might  require  any  considerable  ascent 
on  one  side  and  descent  on  the  other. 

641.  The  gradients  upon  common  roads  will  depend  upon  the 
kind  of  material  used  for  the  road-covering,  and  upon  the  state 
in  which  the  road-surface  is  kept.  The  gradient  in  all  cases 
should  be  less  than  the  angle  of  repose,  or  of  that  inclination  of 
the  axis  of  the  road  in  which  the  ordinary  vehicles  for  transporta 
tion  would  remain  at  a  state  of  rest,  or,  if  placed  in  motion,  would 
descend  by  the  action  of  gravity  with  uniform  velocity. 

The  gradients  corresponding  to  the  angle  of  repose  have  been 
ascertained  by  experiments  made  upon  the  various  road-coverings 
in  ordinary  use,  by  allowing  a  vehicle  to  descend  along  a  road 
of  variable  inclination  until  it  was  brought  to  a  state  of  rest  by 
the  retarding  force  of  friction ;  also,  by  ascertaining  the  amount  of 
force,  termed  the  force  of  traction,  requisite  to  put  in  motion  a 
vehicle  with  a  given  load  on  a  level  road. 

The  following  are  the  results  of  experiments  made  by  Mr. 
Macneill,  in  England,  to  determine  the  force  of  traction  for  one 
ton  upon  level  roads. 
No.  1.  Good  pavement,  the  force  of  traction  is  .       33  lbs 

M    2.  Broken  stone  surface  laid  on  an  old  flint  road       65    " 

"    3.  Gravel  road 147   " 


ROADS.  281 

No.  4.  Broken- stone  surface  on  a  rough  pave  nent 

bottom     .......        16  lbs. 

"    5.  Broken-stone  surface  on  a  bottom  of  beton     .       46    " 

From  this  it  appears  that  the  angle  of  repose  in  the  first  case 
is  represented  by  affo*  or  eV  nearly;  and  that  the  slope  of 
the  road  should  therefore  not  be  greater  than  one  perpendicular 
to  sixty-eight  in  length  ;  or  that  the  height  to  be  overcome  must 
not  be  greater  than  one  sixty-eighth  of  the  distance  between  the 
two  points  measured  along  the  road,  in  order  that  the  force  of 
friction  may  counteract  that  of  gravity  in  the  direction  of  the 
road. 

A  similar  calculation  will  show  that  the  angle  of  repose  in  the 
other  cases  will  be  as  follows  : 
'  No.  2,  .         .         .     1  to     .         .         .35  nearly. 

"3,        .         .         .         .     1  to     .         .         .     15      " 
"    4  and  5,       .  .  .     1  to     .  .  .     49      " 

These  numbers,  which  give  the  angle  of  repose  between  ^j 
and  TV  for  the  kinds  of  road-covering  Nos.  2  and  4  in  most  or- 
dinary use,  and  corresponding  to  a  road-surface  in  good  order, 
may  be  somewhat  increased,  to  from  aV  to  ^3,  for  the  ordinary 
state  of  the  surface  of  a  well-kept  road,  without  there  being  any 
necessity  for  applying  a  brake  to  the  wheels  in  descending,  or 
going  out  of  a  trot  in  ascending.  The  steepest  gradient  that  can 
be  allowed  on  roads  with  a  broken-stone  covering  is  about  ^\,  as 
this,  from  experience,  is  found  to  be  about  the  angle  of  repose 
upon  roads  of  this  character  in  the  state  in  which  they  are  usually 
kept.  Upon  a  road  with  this  inclination,  a  horse  can  draw  at  a 
walk  his  usual  load  for  a  level  without  requiring  the  assistance 
of  an  extra  horse  ;  and  experience  has  farther  shown  that  a  horse 
at  the  usual  walking  pace  will  attain,  with  less  apparent  fatigue, 
the  summit  of  a  gradient  of  fa  in  nearly  the  same  time  that  he 
would  require  to  reach  the  same  point  on  a  trot  over  a  gradient 
of  fa- 

A  road  on  a  dead  level,  or  one  with  a  continued  and  uniform 
ascent  between  the  points  of  arrival  and  departure,  where  they  lie 
upon  different  levels,  is  not  the  most  favorable  to  the  draft  of  the 
horse.  Each  of  these  seems  to  fatigue  him  more  than  a  line  of 
aiternate  ascents  and  descents  of  slight  gradients ;  as,  for  exam- 
pie,  gradients  of  T|ff,  upon  which  a  horse  will  draw  as  heavy  a 
load  with  the  same  speed  as  upon  a  horizontal  road. 

The  gradients  should  in  all  cases  be  reduced  as  far  as  prac- 
ticable, as  the  extra  exertion  that  a  horse  must  put  forth  in  over- 
coming heavy  gradients  is  very  considerable  ;  they  should  as  a 
general  rule,  therefore,  be  kept  as  low  at  least  as  3'^,  wherever 
the  ground  will  admit  of  it.  This  can  generally  be  effected,  even 
in  ascending  steep  hill-sides,  by  giving  the  axis  of  the  road  a  zig. 

36 


2»2  ROADS. 

zag  direction,  connecting  the  straight  portions  of  the  zigzags  by 
circulai  arcs.  The  grad  ents  of  the  curved  portions  of  the  zig- 
zags should  be  rec'uced,  and  the  roadway  also  at  these  points  be 
widened,  for  the  safety  of  vehicles  descending  rapidly.  The 
width  of  the  roadway  may  be  increased  about  one  fourth,  when 
the  angle  between  the  straight  portions  of  the  zigzags  *s  from 
120°  to  90°;  and  the  increase  should  be  nearly  one  half  where 
the  angle  is  from  90°  to  60°. 

642.  Having  laid  down  upon  the  map  the  approximate  location 
of  the  axis  of  the  road,  a  comparison  can  then  be  made  between 
the  solid  contents  of  the  excavations  and  embankments,  which 
should  be  so  adjusted  that  >hey  shall  balance  each  other,  or,  in 
other  words,  the  necessary  excavations  shall  furnish  sufficient 
earth  to  form  the  embankments.  To  effect  this,  it  will  frequently 
be  necessary  to  alter  the  first  location,  bv  shifting  the  position  of 
the  axis  to  the  right  or  left  of  the  position  first  assumed,  and  alsa 
by  changing  the  gradients  within  the  prescribed  limits.  This 
is  a  problem  of  very  considerable  intricacy,  whose  solution  can 
only  be  arrived  at  by  successive  approximations.  For  this  pur- 
pose, the  line  must  be  subdivided  into  several  portions,  in  each 
of  which  the  equalization  should  be  attempted  independently  of 
the  rest,  instead  of  trying  a  general  equalization  for  the  whole 
line  at  once. 

In  the  calculations  of  solid  contents  required  in  balancing  the 
excavations  and  embankments,  the  most  accurate  method  consists 
in  subdividing  the  different  solids  into  others  of  the  most  simple 
geometrical  forms,  as  prisms,  prismoids,  wedges,  and  pyramids, 
whose  solidities  are  readily  determined  by  the  ordinary  rules  for 
the  mensuration  of  solids.  As  this  process,  however,  is  frequently 
long  and  tedious,  other  methods  requiring  less  time  but  not  so 
accurate,  are  generally  preferred,  as  their  results  give  an  approx- 
imation sufficiently  near  the  true  for  most  practical  purposes. 
They  consist  in  taking  a  number  of  equidistant  profiles,  and  cal- 
culating the  solid  contents  between  each  pair,  either  by  multiply- 
ing the  half  sum  of  their  areas  by  the  distance  between  them,  or 
else  by  taking  the  profile  at  the  middle  point  between  each  pair, 
and  multiplying  its  area  by  the  same  length  as  before.  The 
latter  method  is  the  more  expeditious  ;  it  gives  less  than  the  true 
solid  contents,  but  a  nearer  approximation  than  the  former,  which 
gives  more  than  the  true  solid  contents,  whatever  may  be  the 
form  of  the  ground  between  each  pair  of  cross  profiles. 

In  calculating  the  solid  contents,  allowance  must  be  made  for 
the  difference  in  bulk  between  the  different  kinds  of  earth  when 
occupying  their  natural  bed  and  when  made  into  embankment 
From  some  careful  experiments  on  this  point  made  by  Mr.  Elwood 
Morris,  a  civil  ergineer,  and  published  in  the  Franklin  Journai 


ROADS.  283 

it  appears  that  light  s  mdy  earth  occupies  the  same  space  both  in 
excavation  and  embankment ;  clayey  earth  about  one  tenth  less 
in  embankment  than  in  its  natural  bed  ;  gravelly  earth  also  about 
one  twelfth  less  ;  rock  in  large  fragments  about  five  twelfth? 
more,  and  in  small  fragments  about  six  tenths  more. 

643.  Another  problem  connected  with  the  one  in  question,  is 
that  of  determining  the  lead,  or  the  mean  distance  to  which  the 
earth  taken  from  the  excavations  must  be  carried  to  form  the 
embankments.  From  the  manner  in  which  the  earth  is  usually 
transported  from  the  one  to  the  other,  this  distance  is  usually  that 
between  the  centre  of  gravity  of  the  solid  of  excavation  and 
that  of  its  corresponding  embankment.  Whatever  disposition 
may  be  made  of  the  solids  of  excavation,  it  is  important,  so  far 
as  the  cost  of  their  removal  is  concerned,  that  the  lead  should  be 
the  least  possible.  The  solution  of  the  problem  under  this  point 
of  view  will  frequently  be  extremely  intricate,  and  demand  the 
application  of  all  the  resources  of  the  higher  analysis.  One  gen- 
eral principle  however  is  to  be  observed  in  all  cases,  in  order  to 
obtain  an  approximate  solution,  which  is,  that  in  the  removal  of 
the  different  portions  of  the  solid  of  excavation  to  their  corre- 
sponding positions  on  that  of  the  embankment,  the  paths  passed 
over  by  their  respective  centres  of  gravity  shall  not  cross  each 
other  either  in  a  horizontal,  or  vertical  direction.  This  may  in 
most  cases  be  effected  by  intersecting  the  solids  of  excavation 
and  embankment  by  vertical  planes  in  the  direction  of  the  re- 
moval, and  by  removing  the  partial  solids  between  the  planes 
within  the  boundaries  marked  out  by  them. 

644.  The  definitive  location  having  been  settled  by  again  going 
over  the  line,  and  comparing  the  features  of  the  ground  with  the 
results  furnished  by  the  preceding  operations,  general  and  de- 
tailed maps  of  the  different  divisions  of  the  definitive  location  are 
prepared,  which  should  give,  with  the  utmost  accuracy,  the  lon- 
gitudinal and  cross  sections  of  the  natural  ground,  and  of  the  ex- 
cavations and  embankments,  with  the  horizontal  and  vertical 
measurements  carefully  written  upon  them,  so  that  the  superin- 
tending engineer  may  have  no  difficulty  in  setting  out  the  work 
from  them  on  the  ground. 

In  addition  to  these  maps,  which  are  mainly  intended  to  guide 
the  engineer  in  regulating  the  earth-work,  detailed  drawings  of  the 
road-covering,  of  the  masonry  and  carpentry  of  the  bridges,  cul- 
verts, &c,  accompanied  by  written  specifications  of  the  manner 
in  which  the  various  kind  of  work  is  to  be  performed,  should  be 
prepared  for  the  guidance  both  of  the  engineer  and  workmen. 

645.  With  the  data  furnished  by  the  maps  and  drawings,  the 
engineer  can  proceed  to  set  out  the  line  on  the  ground.  ■  The 
axis  of  the  road  is  determined  by  placing  stout  stakes,  or  picket* 


284  ROADS. 

at  equal  intervals  apart,  which  are  numbered  to  correspond  with 
the  same  points  on  the  map.  The  width  of  the  roadway  and  the 
lines  on  the  ground  corresponding  to  the  side  slopes  of  the  exca- 
vations and  embankments,  are  laid  out  in  the  same  manner,  by 
stakes  placed  along  the  lines  of  the  cross  profiles. 

Besides  the  numbers  marked  on  the  stakes,  to  indicate  their 
position  on  the  map,  other  numbers,  showing  the  depth  of  the 
excavations,  or  the  height  of  the  embankments  from  the  surface 
of  the  ground,  accompanied  by  the  letters  Cut.  Fill,  to  indicate  a 
cutting,  or  a.  filling,  as  the  case  may  be,  are  also  added  to  guide 
the  workmen  in  their  operations.  The  positions  of  the  stakes  on 
the  ground,  which  show  the  principal  points  of  the  axis  of  the 
road,  should,  moreover,  be  laid  down  on  the  map  with  great  ac- 
curacy, by  ascertaining  their  bearings  and  distances  from  any  fixed 
and  marked  objects  in  their  vicinity,  in  order  that  the  points  may 
be  readily  found  should  the  stakes  be  subsequently  misplaced. 

646.  Earth-work.  This  term  is  applied  to  whatever  relates  to 
the  construction  of  the  excavations  and  embankments,  to  prepare 
them  for  receiving  the  road-covering. 

647.  In  forming  the  excavations,  the  inclination  of  the  side 
slopes  demands  peculiar  attention.  This  inclination  will  depend 
on  the  nature  of  the  soil,  and  the  action  of  the  atmosphere  and 
internal  moisture  upon  it.  In  common  soils,  as  ordinary  garden 
earth  formed  of  a  mixture  of  clay  and  sand,  compact  clay,  and 
compact  stony  soils,  although  the  side  slopes  would  withstand 
very  well  the  effects  of  the  weather  with  a  greater  inclination,  it 
is  best  to  give  them  two  base  to  one  perpendicular ;  as  the  sur- 
face of  the  roadway  will,  by  this  arrangement,  be  well  exposed 
to  the  action  of  the  sun  and  air,  which  will  cause  a  rapid  evapo- 
ration of  the  moisture  on  the  surface.  Pure  sand  and  gravel  may 
require  a  greater  slope,  according  to  circumstances.  In  all  cases 
where  the  depth  of  the  excavation  is  great,  the  base  of  the  slope 
should  be  increased.  It  is  not  usual  to  use  any  artificial  means 
to  protect  the  surface  of  the  side  slopes  from  the  action  of  the 
weather ;  but  it  is  a  precaution  which,  in  the  end,  will  save  much 
labor  and  expense  in  keeping  the  roadway  in  good  order.  The 
simplest  means  which  can  be  used  for  this  purpose,  consist  in  cov- 
ering the  slopes  with  good  sods,  (Fig.  149,)  or  else  with  a  layer 

Fig.  149 — CrosB  section  of  a  roac 
in  excavation. 

A,  road-surface. 

B,  side  slopes. 

C,  top  surface-drain. 

of  vegetable  mould  about  four  inches  thick,  carefully  laid  and 
sown' with  grass  seed.  These  means  will  be  amply  sufficient  to 
protect  the  side  slopes  from  injury  when  they  are  not  exposed  to 


ROADS.  285 

any  other  causes  of  deterioration  than  the  wash  of  the  rain,  and 
the  action  of  frost  on  the  ordinary  moisture  retained  by  the  soil. 

The  side  slopes  form  usually  an  unbroken  surface  from  the 
foot  to  the  top.  But  in  deep  excavations,  and  particularly  in  soils 
liable  to  slips,  they  are  sometimes  formed  with  horizontal  offsets, 
termed  benches,  which  are  made  a  few  feet  wide  and  have  a  ditch 
on  the  inner  side  to  receive  the  surface-water  from  the  portion  of 
the  side  slope  above  them.  These  benches  catch  and  retain  the 
earth  that  may  fall  from  the  portion  of  the  side  slope  above. 

When  the  side  slopes  are  not  protected,  it  will  be  well,  in  lo- 
calities where  stone  is  plenty,  to  raise  a  small  wall  of  dry  stone 
at  the  foot  of  the  slopes,  to  prevent  the  wash  of  the  slopes  from 
being  carried  into  the  roadway. 

A  covering  of  brush  wood,  or  a  thatch  of  straw,  may  also  be 
used  with  good  effect ;  but,  from  their  perishable  nature,  they 
will  require  frequent  renewal  and  repairs. 

In  excavations  through  solid  rock,  which  does  not  disintegrate 
on  exposure  to  the  atmosphere,  the  side  slopes  might  be  made 
perpendicular ;  but  as  this  would  exclude,  in  a  great  degree,  the 
action  of  the  sun  and  air,  which  is  essential  to  keeping  the  road- 
surface  dry  and  in  good  order,  it  will  be  necessary  to  make  the 
side  slopes  with  an  inclination,  varying  from  one  base  to  one 
perpendicular,  to  one  base  to  two  perpendicular,  or  even  greater, 
according  to  the  locality ;  the  inclination  of  the  slope  on  the 
south  side  in  northern  latitudes  being  greatest,  to  expose  better 
the  road-surface  to  the  sun's  rays. 

The  slaty  rocks  generally  decompose  rapidly  on  the  surface, 
when  exposed  to  moisture  and  the  action  of  frost.  The  side 
slopes  in  rocks  of  this  character  may  be  cut  into  steps,  (Fig.  150,) 


and  then  be  covered  by  a  layer  of  vegetable  mould  sown  in  grass 
seed,  or  else  the  earth  may  be  sodded  in  the  usual  way. 

648.  The  stratified  soils  and  rocks,  in  which  the  strata  have  a 
dip,  or  inclination  to  the  horizon,  are  liable  to  slips,  or  to  give 
A'ay  by  one  stratum  becoming  detached  and  sliding  on  another 
which  is  caused  either  from  the  action  of  frost,  or  from  the  pres- 
sure of  water,  which  insinuates  itself  between  the  strata.  The  worst 
soils  of  this  character  are  those  formed  of  alternate  strata  of  clay 
and  sand  ;  particularly  if  the  clay  is  of  a  nature  to  become  semi- 
fluid when  mixed  with  water.     The  best  preventives  that  can  be 


886  ROADS. 

resorted  to  in  these  cases,  are  to  adopt  a  thorough  system  of 
drainage,  to  prevent  the  surface-water  of  the  ground  from  run 
ning  down  the  side  slopes,  and  to  cut  off  all  springs  which  rur. 
towards  the  roadway  from  the  side  slopes.  The  surface-watei 
may  be  cut  off  by  means  of  a  single  tlitch  (Fig.  149)  made  on 
the  up-hill  side  of  the  road,  to  catch  the  water  before  it  reaches 
the  slope  of  the  excavation,  and  convey  it  off  to  the  natural 
water-courses  most  convenient :  as,  in  almost  every  case,  it  wiP 
be  found  that  the  side  slope  on  the  down-hill  side  is,  compara 
tively  speaking,  but  slightly  affected  by  the  surface-water. 

Where  slips  occur  from  the  action  of  springs,  it  frequently 
becomes  a  very  difficult  task  to  secure  the  side  slopes.  If  the 
sources  can  be  easily  reached  by  excavating  into  the  side  slopes, 
drains  formed  of  layers  of  fascines,  or  brush-wood,  may  be  placed 
to  give  an  outlet  to  the  water,  and  prevent  its  action  upon  the 
side  slopes.  The  fascines  may  be  covered  on  top  with  good 
sods  laid  with  the  grass  side  beneath,  and  the  excavation  made 
to  place  the  drain  be  filled  in  with  good  earth  well  rammed. 
Drains  formed  of  broken  stone,  covered  in  like  manner  on  top 
with  a  layer  of  sod  to  prevent  the  drain  from  becoming  choked 
with  earth,  maybe  used  under  the  same  circumstances  as  fascine 
drains.  Where  the  sources  are  not  isolated,  and  the  whole  mass 
of  the  soil  forming  the  side  slopes  appears  saturated,  the  drainage 
may  be  effected  by  excavating  trenches  a  few  feet  wide  at  inter- 
vals to  the  depth  of  some  feet  into  the  side  slopes,  and  filling 
them  with  broken  stone,  or  else  a  general  drain  of  broken  stone 
may  be  made  throughout  the  whole  extent  of  the  side  slope  by 
excavating  into  it.  When  this  is  deemed  necessary,  it  will  be 
well  to  arrange  the  drain  like  an  inclined  retaining-wall,  with 
buttresses  at  intervals  projecting  into  the  earth  farther  than  the 
general  mass  of  the  drain.  The  front  face  of  the  drain  should,  in 
this  case,  also  be  covered  with  a  layer  of  sods  with  the  grass  side 
beneath,  and  upon  this  a  layer  of  good  earth  should  be  compactly 
laid  to  form  the  face  of  the  side  slopes.  The  drain  need  only  be 
carried  high  enough  above  the  foot  of  the  side  slope  to  tap  all  the 
sources  ;  and  it  should  be  sunk  sufficiently  below  the  roadway- 
surface  to  give  it  a  secure  footing. 

The  drainage  has  been  effected,  in  some  cases,  by  sinking 
.wells  or  shafts  at  some  distance  behind  the  side  slopes,  from  the 
top  surface  to  the  level  of  the  bottom  of  the  excavation,  and  lead- 
ing the  water  which  collects  in  them  by  pipes  into  drains  at  the 
foot  of  the  side  slopes.  In  others  a  narrow  trench  has  been  ex- 
cavated, parallel  to  the  axis  of  the  road,  from  the  top  surface  to 
a  sufficient  depth  to  tap  all  the  sources  which  flow  towards  the 
side  slope,  and  a  drain  formed  either  by  filling  the  trench  wholly 
with  broken  stone,  or  else  by  arranging  an  open  conduit  at  the 


ROADS  28*3 

bottom  to  receive  the  water  collected,  over  which  a  layer  of 
brushwood  is  laid,  the  remainder  of  the  irench  being  filled  with 
broken  stone. 

In  some  recent  instances  in  England,  the  side  slopes  of  very 
bad  soils  have  been  secured  by  a  facing  of  brick  arranged  in  a 
manner  very  similar  to  the  method  resorted  to  for  securing  the 
perpendicular  sides  of  narrow  deep  trenches  by  a  timber- facing. 
The  plan  pursued  is  to  place,  at  intervals  along  the  excavation, 
strong  buttresses  of  brick  on  each  side,  opposite  to  each  other, 
and  to  connect  them  at  bottom  by  a  reversed  arch.  Between 
these  buttresses  are  placed,  at  suitable  heights,  one  or  more  brick 
beams,  formed  at  bottom  with  a  fiat  segment  arch,  and  at  top 
with  a  like  inverted  arch.  The  buttresses,  secured  in  this  way, 
serve  as  piers  for  vertical  cylindrical  arches,  which  form  the 
facing  and  support  the  pressure  of  the  earth  between  the  but- 
tresses. 

649.  In  forming  the  embankments,  (Fig.  151,)  the  side  slopes 

a  _•   » 


should  be  made  with  a  less  inclination  than  that  which  the  earth 
naturally  assumes  ;  for  the  purpose  of  giving  them  greater  dura- 
bility, and  to  prevent  the  width  of  the  top  surface,  along  which 
the  roadway  is  made,  from  diminishing  by  every  change  in  the 
side  slopes,  as  it  would  were  they  made  with  the  natural  slope 
To  protect  the  side  slopes  more  effectually,  they  should  be  sod 
ded,  or  sown  in  grass  seed  ;  and  the  surface-water  of  the  lop 
should  not  be  allowed  to  run  down  them,  as  it  would  soon  wash 
them  into  gullies,  and  destroy  the  embankment.  In  localities 
where  stone  is  plenty,  a  sustaining  wall  of  dry  stone  may  be  ad- 
vantageously substituted  for  the  side  slopes. 

To  prevent,  as  far  as  possible,  the  settling  which  takes  place 
in  embankments,  they  should  be  formed  with  great  care;  the 
earth  being  laid  in  successive  layers  of  about  four  feet  in  thick- 
ness, and  each  layer  well  settled  with  rammers.  As  this  method 
is  very  expensive,  it  is  seldom  resorted  to  except  in  works  which 
require  great  care,  and  are  of  trifling  extent.  For  extensive 
works,  the  method  usually  followed  on  account  of  economy,  is* 
to  embank  out  from  one  end,  carrying  forward  the  work  on  a 
level  with  the  top  surface.  In  this  case,  as  there  must  be  a  want 
of  compactness  in  the  mass,  it  would  be  best  to  form  the  outsides 
of  the  embankment  first,  and  to  gradually  fill  in  towards  the  cen- 
tre, in  order  that  the  earth  may  arrange  itself  in  layers  with  a  dip 
from  the  sides  inwards :  this  will  in  a  great  measure  counteract 


288 


ROADS. 


any  tendency  to  slips  outward.  The  foot  of  the  slopes  should 
be  secured  by  buttressing  them  either  by  a  low  stone  wall,  ol 
by  forming  a  slight  excavation  for  the  same  purpose. 

650.  When  the  axis  of  the  roadway  is  laid  out  on  the  side 
slope  of  a  hill,  and  the  road-surface  is  formed  partly  by  excava- 
ting and  partly  by  embanking  out,  the  usual  and  most  simple 
method  is  to  extend  out  the  embankment  gradually  along  the 
whole  line  of  excavation.  This  method  is  insecure,  and  no  pains 
therefore  should  be  spared  to  give  the  embankment  a  good  foot- 
ing on  the  natural  surface  upon  which  it  rests,  particularly  at  the 
foot  of  the  slope.    For  this  purpose  the  natural  surface  (Fig.  152) 


should  be  cut  into  steps,  or  offsets,  and  the  foot  of  the  slope  be 
secured  by  buttressing  it  against  a  low  stone  wall,  or  a  small 
terrace  of  carefully  rammed  earth. 

In  side-formings  along  a  natural  surface  of  great,  inclination, 
the  method  of  construction  just  explained  will  not  be  sufficiently 
secure  ;  sustaining-walls  must  be  substituted  for  the  side  slopes, 
both  of  the  excavations  and  embankments.  These  walls  may  be 
made  simply  of  dry  stone,  when  the  stone  can  be  procured  in 
blocks  of  sufficient  size  to  render  this  kind  of  construction  of 
sufficient  stability  to  resist  the  pressure  of  the  earth.  But 
when  the  blocks  of  stone  do  not  offer  this  security,  they  must 


Fig.  153— Cross  section  of  a  road  in  steep 
side-forming. 

A,  rilling. 

B,  sustaining- wall  of  filling. 

C,  breast-wall  of  cutting. 

D,  parapet-wall  of  footpath. 


oe  laid  in  mortar,  (Fig.  153,)  and  hydraulic  mortar  is  the  011I7 


ROADS.  289 

Kind  which  will  form  a  safe  construction.  The  wall  which  sup 
plie#  the  slope  of  the  excavation  should  be  carried  up  as  high  as 
the  natural  surface  of  the  ground ;  the  one  that  sustains  the  em- 
bankment should  be  built  up  to  the  surface  of  the  roadway  ;  and 
a  parapet-wall  should  be  raised  upon  it,  to  secure  vehicles  from 
accidents  in  deviating  from  the  line  of  the  roadway. 

A  road  may  be  constructed  partly  in  excavation  and  partly  in 
embankment  along  a  rocky  ledge,  by  blasting  the  rock,  when  ihe 
inclination  of  the  natural  surface  is  not  greater  than  one  perpen- 
dicular to  two  base  ;  but  with  a  greater  inclination  than  this,  the 
whole  should  be  in  excavation. 

651.  There  are  examples  of  road  constructions,  i;  .ocalities 
like  the  last,  supported  on  a  frame-work,  consisting  of  horizontal 
pieces,  which  are  firmly  fixed  at  one  end  by  being  let  into  holes 
drilled  in  the  rock,  and  are  sustained  at  the  other  by  an  inclined 
strut  underneath,  which  rests  against  the  rock  in  a  shoulder 
formed  to  receive  it. 

652.  When  the  excavations  do  not  furnish  sufficient  earth  for 
the  embankments,  it  is  obtained  from  excavations,  termed  side- 
cuttings,  made  some  place  in  the  vicinity  of  the  embankment, 
from  which  the  earth  can  be  obtained  with  the  most  economy. 

If  the  excavations  furnish  more  earth  than  is  required  for  the 
embankment,  it  is  deposited  in  what  is  termed  spoil-bank,  on  the 
side  of  the  excavation.  The  spoil-bank  should  be  made  at  some 
distance  back  from  the  side  slope  of  the  excavation,  and  on  the 
down-hill  side  of  the  top  surface  ;  and  suitable  drains  should  br 
arranged  to  carry  off  any  water  that  might  collect  near  it  and  af 
feet  the  side  slope  of  the  excavation. 

The  forms  to  be  given  to  side-cuttings  and  spoil-banks  will 
depend,  in  a  great  degree,  upon  the  locality  :  they  should,  as  far 
as  practicable,  be  such  that  the  cost  of  removal  of  the  earth  shal' 
oe   east  possible 

653.  Drainage.  A  system  of  thorough  drainage,  by  whicn 
.ne  water  that  niters  through  the  ground  will  be  cut  off  from  the 
toil  beneath  the  roadway,  to  a  depth  of  at  least  three  feet  below 
lie  bottom  of  the  road-covering,  and  by  which  that  which  falls 
upon  the  surface  will  be  speedily  conveyed  off,  before  it  can  filter 
through  the  road-covering,  is  essential  to  the  good  condition  of  a 
road. 

The  surface-water  is  conveyed  off  by  giving  the  surface  of  the 
roadway  a  slight  transverse  convexity,  from  the  middle  to  the 
sides,  where  the  water  is  received  into  the  gutters,  or  side  chan- 
nels, from  which  it  is  conveyed  by  underground  aqueducts,  termed 
culverts,  built  of  stone  or  brick  and  usually  arched  at  top.  into 
he  main  drains  that  communicate  with  the  natural  water-courses. 
This  convexity  is  regulated  by  making  the  figure  of  the  profile 

37 


290 


ROADS. 


an  ellipse,  of  which  the  semi-transverse  axis  is  15  feet,  and  the 
semi-conjugate  axis  9  inches ;  thus  placing  the  middle  of  the 
roadway  nine  inches  above  the  bottom  of  the  side  channels.  This 
convexity,  which  is  as  great  as  should  be  given,  will  not  be  sufh" 
cient  in  a  flat  countiy  to  keep  the  road-surface  dry ;  and  in  such 
localities,  if  a  slight  longitudinal  slope  cannot  be  given  to  the 
road,  it  should  be  raised,  when  practicable,  three  or  four  feet 
above  the  general  level ;  both  on  account  of  conveying  off  speedily 
the  surface-water,  and  exposing  the  surface  better  to  the  action 
of  the  wind. 

To  drain  the   soil  beneath  the  roadway  in  a  level  country, 
ditches,  termed  open  side  drains,  (Fig.  154,)  are  made  parallel 


Fig.  154 — Cross  section  of  broken-stone  road-covering. 

A,  road-surface. 

B,  side  channels. 

C,  footpath. 

D,  covered  drains,  or  culverts,  leading  from  side  channels  to  the  side  drains  h. 

to  the  road,  and  at  some  feet  from  it. on  each  side.  The  bottom 
of  the  side  drains  should  be  at  least  three  feet  below  the  road- 
covering  ;  their  size  will  depend  on  the  nature  of  the  soil  to  be 
drained.  In  a  cultivated  country  the  side  drains  should  be  on  the 
field  side  of  the  fences. 

As  open  drains  would  be  soon  filled  along  the  parts  of  a  road 
in  excavation,  by  the  washings  from  the  side  slopes,  covered 
drains,  built  either  of  brick  or  stone,  must  be  substituted  for 
them.     These  drains  (Fig.  155)  consist  simply  of  a  flooring  of 


Fig.  155 — Cross  section  of  a  covered  dram. 
A,  drain. 

a,  a,  side  walls. 

b,  top  stones. 

c,  bottom  stones. 

(/,  broken  stone  or  large  gravel  laid  over  brush. 


flagging  stone,  or  of  bncK,  with  two  side  walls  of  rubble,  or  brick 
masonry,  which  support  a  top  covering  of  flat  stones,  or  of  brick, 
with  open  joints,  of  about  half  an  inch,  to  give  a  free  passage 
way  to  the  water  into  the  drain.  The  top  is  covered  with  a  layer 
of  straw  or  brushwood  ;  and  clean  gravel,  or  broken  stone,  in 
small  fragments,  is  laid  over  this,  for  the  purpose  of  allowing  the 


ROADS.  291 

water  to  filter  freely  through  to  the  drain,  without  carrying  with  it 
any  earth  or  sediment,  which  might  in  time  accumulate  and  choke 
it.  The  width  and  height  of  covered  dra.r.s  will  depend  on  the 
materials  of  which  they  are  built,  and  the  quantity  of  water  to 
which  they  yield  a  passage. 

Besides  the  longitudinal  covered  drains  in  cuttings,  other  drains 
are  made  under  the  roadway  which,  from  their  form,  are  termed 
cross  mitre  drains.  Their  plan  is  in  shape  like  the  letter  V,  the 
angular  point  being  at  the  centre  of  the  road,  and  pointing  in  the 
direction  of  its  ascent.  The  angle  should  be  so  regulated  that 
the  bottom  of  the  drain  shall  not  have  a  greater  slope  along  either 
of  its  branches,  than  one  perpendicular  to  one  hundred  base,  to  pre- 
serve the  masonry  from  damage  by  the  current  The  construc- 
tion of  mitre  drains  is  the  same  as  the  covered  longitudinal  drains. 
They  should  be  placed  at  intervals  of  about  60  yards  from  each 
other 

In  some  cases  surface  drains,  termed  catch-water  drains,  are 
made  on  the  side  slopes  of  cuttings.  They  are  run  up  obliquely 
along  the  surface,  and  empty  directly  into  the  cross  drains  which 
convey  the  water  into  the  natural  water-courses. 

When  the  roadway  is  in  side-forming,  cross  drains  of  the  or- 
dinary form  of  culverts  are  made,  to  convey  the  water  from  the 
side  channels  and  the  covered  drains  into  the  natural  water- 
courses. They  should  be  of  sufficient  dimensions  to  convey  off 
a  large  volume  of  water,  and  to  admit  a  man  to  pass  through 
them  so  that  they  may  be  readily  cleared  out,  or  even  repaired, 
without  breaking  up  the  roadway  over  them. 

The  only  drains  required  for  embankments  are  the  ordinary 
side  channels  of  the  roadway,  with  occasional  culverts,  to  convey 
die  water  from  them  into  the  natural  water-courses.  Great  care 
should  be  taken  to  prevent  the  surface-water  from  running  down 
the  side  slopes,  as  they  would  soon  be  washed  into  gullies  by  it. 

Very  wet  and  marshy  soils  require  to  be  thoroughly  drained 
before  the  roadway  can  be  made  with  safety.  The  best  system 
that  can  be  followed  in  such  cases,  is  to  cut  a  wide  and  deep  open 
main-drain  on  each  side  of  the  road,  to  convey  the  water  to  the 
natural  water-courses.  Covered  cross  drains  should  be  made  at 
frequent  intervals,  to  drain  the  soil  under  the  roadway.  They 
should  be  sunk  as  low  as  will  admit  of  the  water  running  from 
them  into  the  main  drains,  by  giving  a  slight  slope  to  the  bottom 
each  way  from  the  centre  of  the  road  to  facilitate  its  flow 

Independently  of  the  drainage  for  marshy  soils,  they  will  re- 
quire, when  the  subsoil  is  of  a  spongy,  elastic  nature,  an  artificial 
bed  for  the  road-covering.  This  bed  may,  in  some  cases,  be 
formed  by  simply  removing  the  upper  stratum  to  a  depth  of  sev- 
eral feet,  and  supplying  its  place  with  well-packed  gravel,  or  any 


292  ROADS. 

soil  of  a  firm  character.  In  other  cases,  when  the  subsoil  yields 
readily  to  the  ordinary  pressure  that  the  road-surface  must  bear, 
a  bed  of  brushwood,  from  9  to  18  inches  in  thickness,  must  be 
formed  to  receive  the  soil  on  which  the  road-covering  is  to  rest. 
The  brushwood  should  be  carefully  selected  from  the  long  straight 
slender  shoots  of  the  branches  or  undergrowth,  and  be  tied  up  in 
bundles,  termed  fascines,  from  9  to  12  inches  in  diameter,  and 
from  10  to  20  feet  long.  The  fascines  are  laid  in  alternate  layers 
crosswise  and  lengthwise,  and  the  layers  are  either  connected  by 
pickets,  or  else  the  withes,  with  which  the  fascines  are  bound, 
are  cut  to  allow  the  brushwood  to  form  a  uniform  and  compact 
bed. 

This  method  of  securing  a  good  bed  for  structures  on  a  weak 
wet  soil  has  been  long  practised  in  Holland,  and  experience  has 
fully  tested  its  excellence. 

654.  Road-coverings.  The  object  of  a  road-covering  being 
to  diminish  the  resistances  arising  from  collision  and  friction, 
and  thereby  to  reduce  the  force  of  traction  to  the  least  prac- 
ticable amount,  it  should  be  composed  of  hard  and  durable  ma- 
terials, laid  on  a  firm  foundation,  and  present  a  uniform  even 
surface. 

The  material  in  ordinary  use  for  road-coverings  is  stone,  eithei 
in  the  shape  of  blocks  of  a  regular  form,  or  of  large  round  peb- 
bles, termed  a  pavement ;  or  broken  into  small  angular  masses  ; 
or  in  the  form  of  gravel. 

655.  Pavements.  The  pavements  in  most  general  use  in  our 
country  are  constructed  of  rounded  pebbles,  known  as  paving 
stones,  varying  from  3  to  8  inches  in  diameter,  which  are  set  in  a 
form,  or  bed  of  clean  sand  or  gravel,  a  foot  or  two  in  thickness, 
which  is  laid  upon  the  natural  soil  excavated  to  receive  the  form. 
The  largest  stones  are  placed  in  the  centre  of  the  roadway.  The 
stones  are  carefully  set  in  the  form,  in  close  contact  with  each 
other,  and  are  then  firmly  settled  by  a  heavy  rammer  until  their 
tops  are  even  with  the  general  surface  of  the  roadway,  which 
should  be  of  a  slightly  convex  shape,  having  a  slope  of  about  ?'T 
from  the  centre  each  way  to  the  sides.  After  the  stones  are 
driven,  the  road-surface  is  covered  with  a  layer  of  clean  sand,  or 
fine  gravel,  two  or  three  inches  in  thickness,  which  is  gradually 
worked  in  between  the  stones  by  the  combined  action  of  the 
travel  over  the  pavement  and  of  the  weather 

The  defects  of  pebble  pavements  are  obvious,  and  confirmed 
by  experience.  The  form  of  sand  or  gravel,  as  usually  made,  is 
not  sufficiently  firm ;  it  should  be  made  in  separate  layers  of 
about  4  inches,  each  layer  being  moistened  and  well  settled  either 
by  ramming,  or  passing  a  heavy  roller  over  it.  Upon  the  form 
prepared  in  this  way  a  layer  of  loose  material  of  two  or  three 


ROADS.  293 

mches  in  thickness  may  be  placed,  to  receive  the  ends  of  the 
paving  stones.  From  the  form  of  the  pebbles,  the  resistance  to 
traction  arising  from  collision  and  friction  is  very  great. 

Pavements  termed  stone  trattways  have  been  tried  in  some  cf 
the  cities  of  Europe,  both  for  light  and  heavy  traffic.  They  are 
formed  by  laying  two  lines  of  long  stone  blocks  for  the  wheels  to 
run  on,  with  a  pavement  of  pebble  for  the  horse-track  between 
the  wheel-tracks.  In  crowded  thoroughfares  tramways  offer  but 
few  if  any  advantages,  as  it  is  impracticable  to  confine  the  vehicles 
to  them,  and  when  exposed  to  heavy  traffic  they  wear  into  ruts. 
The  stone  blocks  should  be  carefully  laid  oh  a  very  firm  bottoming, 
and  particular  attention  is  requisite  to  prevent  ruts  from  forming 
between  the  blocks  and  the  pebble  pavement. 

Stone  suitable  for  pavements  should  be  hard  and  tough,  and 
not  wear  smooth  under  the  action  to  which  it  is  exposed.  Some 
varieties  of  granite  have  been  found  in  England  to  furnish  the 
best  paving  blocks.  In  France,  a  very  fine-grained  compact  gray 
sandstone  of  a  bluish  cast  is  mostly  in  use  for  the  same  purpose, 
but  it  wears  quite  smooth. 

The  sand  used  for  forms  should  be  clean  and  free  from  peb- 
bles and  gravel  of  a  larger  grain  than  about  two  tenths  of  an  inch. 
The  form  should  be  made  by  moistening  the  sand,  and  com- 
pressing it  in  layers  of  about  four  inches  in  thickness,  either  by 
ramming,  or  by  passing  over  each  layer  several  times  a  heavy 
iron  roller.  Upon  the  top  layer  about  an  inch  of  loose  sand  may 
be  spread  to  receive  the  blocks  ;  the  joints  between  which,  after 
they  are  placed,  should  be  carefully  filled  with  sand. 

The  sand  form,  when  carefully  made,  presents  a  very  firm  and 
stable  foundation  for  the  pavement. 

Wooden  pavements,  formed  of  blocks  of  wood  of  various 
shapes,  have  been  tried  in  England  and  several  of  our  cities 
within  the  last  few  years,  but  are  now  for  the  most  part  aban- 
doned, as  the  material  has  been  found  to  decay  very  rapidly, 
even  when  prepared  with  some  of  the  preservatives  of  timber 
against  the  rot. 

Asphaltic  pavements  have  undergone  a  like  trial,  and  have  also 
been  found  to  fail  after  a  few  years  service.  This  material  is 
farther  objectionable  as  a  pavement  in  cities  where  the  pave- 
ments and  sidewalks  have  frequently  to  be  disturbed  for  the 
purposes  of  repairing,  or  laying  down  sewers,  water-pipes,  and 
other  necessary  conveniences  for  a  city. 

The  best  system  of  pavement  is  that  which  has  been  partially 
put  in  practice  in  some  of  the  commercial  cities  of  England,  the 
idea  of  which  seems  to  have  been  taken  from  the  excellent  mili 
tary  roads  of  the  Romans,  vestiges  :i  which  remain  at  the  present 
day  in  a  good  state. 


294  ROADS 

In  constructing  this  pavement,  a  bed  (Fig.  156)  is  first  pre 
pared,  by  removing  the  surface  of  the  soil  to  the  depth  of  a  foot 
or  more,  to  obtain  a  firm  stratum ;  the  surface  of  this  bed  re 

B 


Fig.  156 — Paved  road-covering. 

A,  pavement. 
C,  curb-stone. 

B,  flagging  of  side-walk. 

ceives  a  very  slight  convexity,  of  about  two  inches  to  ten  feet, 
from  the  centre  to  the  sides  of  the  roadway.  If  the  soil  is  of  a 
soft  clayey  nature,  into  which  small  fragments  of  broken  stone 
would  be  easily  worked  by  the  wheels  of  vehicles,  it  should  be 
excavated  a  foot  or  two  deeper  to  receive  a  form  of  sand,  or  of 
clean  fine  gravel.  On  the  surface  of  the  bed  thus  prepared,  a 
layer  of  small  broken  stone,  four  inches  thick,  is  laid  ;  the  di- 
mensions of  these  fragments  should  not  be  greater  than  two  and 
a  half  inches  in  any  direction ;  the  road  is  then  opened  to  vehicles 
until  this  first  layer  becomes  perfectly  compact ;  care  being  taken 
to  fill  up  any  ruts  with  fresh  stone,  in  order  to  obtain  a  uniform 
surface.  A  second  layer  of  stone,  of  the  same  thickness  as  the 
first,  is  then  laid  on,  and  treated  in  the  same  manner ;  and  finally 
a  third  layer.  When  the  third  layer  has  become  perfectly  com- 
pact, and  is  of  a  uniform  surface,  a  layer  of  fine  clean  gravel, 
two  and  a  half  inches  thick,  is  spread  evenly  over  it  to  receive 
the  paving  stones.  The  blocks  of  stone  are  of  a  square  shape, 
and  of  different  sizes,  according  to  the  nature  of  the  travelling 
over  the  pavement.  The  largest  size  are  ten  inches  thick,  nine 
inches  broad,  and  twelve  inches  long  ;  the  smallest  are  six  inches 
thick,  five  inches  broad,  and  ten  inches  long.  Each  block  is 
carefully  settled  in  the  form,  by  means  of  a  heavy  beetle ;  it  is 
then  removed  in  order  to  cover  the  side  of  the  one  against  which 
it  is  to  rest  with  hydraulic  mortar ;  this  being  done,  the  block  is 
replaced,  and  properly  adjusted.  The  blocks  of  the  different 
courses  across  the  roadway  should  break  joints.  The  surface  of 
the  road  is  convex ;  the  convexity  being  determined  by  rqaking 
the  outer  edges  six  inches  lower  than  the  middle,  for  a  width  of 
thirty  feet. 

This  system  of  pavement  fulfils  in  the  best  manner  all  the  re- 
quisites of  a  good  road-covering,  presenting  a  hard  even  surface 
to  the  action  of  the  wheels,  and  reposing  on  a  firm  bed  formed 
by  the  broken-stone  bottoming.  The  mortar-joints,  so  long  as 
they  remain  tight,  will  effectually  prevent  the  penetration  of  water 
beneath  the  pavement ;  but  it  is  probable,  from  the  effect  of  the 
transit  of  heavily-laden  vehicles,  and  from  the  •  expansion  and 


R   ADS.  295 

contraction  of  the  stone,  which  in  our  climate  is  found  to  be  verj 
considerable,  that  the  mortar  would  soon  be  crushed  and  washed 
out. 

In  France,  and  in  many  of  the  large  cities  of  the  continent,  the 
pavements  are  made  with  blocks  of  rough  stone  of  a  cubical  form, 
measuring' between  eight  and  nine  inches  along  the  edge  of  the 
cube.  These  are  laid  on  a  form  of  sand  of  only  a  few  inches  thick 
when  the  soil  beneath  is  firm ;  but  in  bad  soils  the  thickness  is 
increased  to  from  six  to  twelve  inches.  The  transversal  joints 
are  usually  continuous,  and  those  in  the  direction  of  the  axis  of 
the  road  break  joints.  In  some  cases  the  blocks  are  so  laid  that 
the  joints  make  an  angle  of  45°  with  the  axis  of  the  roadway,  one 
set  being  continuous,  the  other  breaking  joints  with  them.  By 
this  arrangement  of  the  joints,  it  is  said  that  the  wear  upon  the 
edges  of  the  blocks,  by  which  the  upper  surface  soon  assumes  a 
convex  shape,  is  diminished.  It  has  been  ascertained  by  expe- 
rience, that  the  wear  upon  the  edges  of  the  blocks  is  greatest  at 
the  joints  which  run  transversely  to  the  axis  when  the  blocks  are 
laid  in  the  usual  manner.  From  the  experiments  of  M.  Morin,  to 
ascertain  the  influence  of  the  shape  of  stone  blocks  on  the  force 
of  traction,  it  was  found  that  the  resistance  offered  by  a  pavement 
of  blocks  averaging  from  five  to  six  inches  in  breadth,  measured 
in  the  direction  of  the  axis  of  the  roadway,  and  about  nine  inches 
in  length,  was  less  than  in  one  of  cubical  blocks  of  the  ordinary 
size. 

Pavements  in  cities  must  be  accompanied  by  sidewalks,  and 
crossing-places,  for  foot-passengers.  The  sidewalks  are  made 
of  large  flat  flagging-stone,  at  least  two  inches  thick,  laid  on  a 
form  of  clean  gravel  well  rammed  and  settled.  The  width  of 
the  sidewalks  will  depend  on  the  street  being  more  or  less  fre 
quented  by  a  crowd.  It  would,  in  all  cases,  be  well  to  have  them 
at  least  twelve  feet  wide  ;  they  receive  a  slope,  or  pitch,  of  one 
inch  to  ten  feet,  towards  the  pavement,  to  convey  the  surface- 
water  to  the  side  channels.  The  pavement  is  separated  from  the 
sidewalk  by  a  row  of  long  slabs  set  on  their  edges,  termed  curb- 
stones, which  confine  both  the  flagging  and  paving  stones.  The 
curb-stones  form  the  side*  of  the  side  channels,  and  should  for 
this  purpose  project  six  inches  above  the  outside  paving  stones, 
and  be  sunk  at  least  four  inches  below  their  top  surface ;  they 
should,  moreover,  be  flush  with  the  upper  surface  of  the  side- 
walks, to  allow  the  water  to  run  over  into  the  side  channels,  and 
to  prevent  accidents  which  might  otherwise  happen  from  their 
tripping  persons  passing  in  haste. 

The  crossings  should  be  from  four  to  six  feet  wide,  and  be 
slightly  raised  above  the  general  surface  of  the  pavement,  to  keep 
them  free  from  mud. 


2S6  ROADS. 

656.  Broken-stone  road-covering.  The  ordinary  road-cover 
ing  for  common  roads,  in  use  in  this  country  and  Europe,  is 
formed  of  a  coating  of  stone  broken  into  small  fragments,  which 
is  laid  either  upon  the  natural  soil,  or  upon  a  paved  bottoming 
of  small  irregular  blocks  of  stone.  In  England  these  two  systems 
have  their  respective  partisans  ;  the  one  claiming  the  superiority 
for  road-coverings  of  stone  broken  into  small  fragments,  a  method 
brought  into  vogue  some  years  since  by  Mr.  McAdam,  from  whom 
these  roads  have  been  termed  macadamized ;  the  other  being  the 
plan  pursued  by  Mr.  Telford  in  the  great  national  roads  con- 
structed in  Great  Britain  within  about  the  same  period. 

The  subject  of  road-making  has  within  the  last  few  years  ex- 
cited renewed  interest  and  discussion  among  engineers  in  France ; 
the  conclusion,  drawn  from  experience,  there  generally  adopted 
is,  that  a  covering  alone  of  stone  broken  into  small  fragments  is 
sufficient  under  the  heaviest  traffic  and  most  frequented  roads. 
Some  of  the  French  engineers  recommend,  in  very  yielding 
clayey  soils,  that  either  a  paved  bottoming  after  Telford's  method 
be  resorted  to,  or  that  the  soil  be  well  compressed  at  the  surface 
before  placing  the  road-covering. 

The  paved  bottom  road-covering  on  Telford's  plan  (Fig.  155) 
is  formed  by  excavating  the  surface  of  the  ground  to  a  suitable 
depth,  and  preparing  the  form  for  the  pavement  with  the  precau- 
tions as  for  a  common  pavement.  Blocks  of  stone  of  an  irregu- 
lar pyramidal  shape  are  selected  for  the  pavement,  which,  for  a 
roadway  30  feel  in  width,  should  be  seven  inches  thick  for  the 
centre  of  the  road,  and  three  inches  thick  at  the  sides.  The  base 
of  each  block  should  not  measure  more  than  five  inches,  and  the 
top  not  less  than  four  inches. 

The  blocks  are  set  by  the  hand,  with  great  care,  as  closely  in 
contact  at  their  bases  as  practicable ;  and  blocks  of  a  suitable 
size  are  selected  to  give  the  surface  of  the  pavement  a  slightly 
convex  shape  from  the  centre  outwards.  The  spaces  between 
the  blocks  are  filled  with  chippings  of  stone  compactly  set  with 
a  small  hammer. 

A  layer  of  broken  stone,  four  inches  thick,  is  laid  over  this 
pavement,  for  a  width  of  nine  feet  on  each  side  of  the  centre  ;  no 
fragment  of  this  layer  should  measure  over  two  and  a  half  inches 
in  any  direction.  A  layer  of  broken  stone  of  smaller  dimensions, 
or  of  clean  coarse  gravel,  is  spread  over  the  wings  to  the  same 
depth  as  the  centre  layer. 

The  road-covering,  thus  prepared,  is  thrown  open  to  vehicles 
until  the  upper  layer  has  become  perfectly  compact ;  care  having 
been  taken  to  fill  in  the  ruts  with  fresh  stone,  in  order  to  obtain 
a  uniform  surface.  A  second  layer,  about  two  inches  in  depth, 
Is  then  laid  over  the  centre  of  the  roadway  ;  and  the  wings  re- 


ROADS. 

ceive  also  a  layer  of  new  material  laid  on  to  a  sufficient  thickness 
to  make  the  outside  of  the  roadway  nine  inches  lower  than  the 
centre,  by  giving  a  slight  convexity  to  the  surface  from  the  centre 
outwards.  A  coating  of  clean  coarse  gravel,  one  inch  and  a  half 
thick,  termed  a  binding,  is  spread  over  the  surface,  and  the  road- 
covering  is  then  ready  to  be  thrown  open  to  travelling. 

The  stone  used  for  the  pavement  may  be  of  an  inferior  quality, 
in  hardness  and  strength,  to  that  placed  at  the  surface,  as  it  is  but 
little  exposed  to  the  wear  and  tear  occasioned  by  travelling.  The 
surface-stone  should  be  of  the  hardest  kind  that  can  be  procured. 
The  gravel  binding  is  laid  over  the  surface  to  facilitate  the  trav- 
elling, whilst  the  under  stratum  of  stone  is  still  loose  ;  it  is,  how- 
ever, hurtful,  as,  by  working  in  between  the  broken  stones,  it 
prevents  them  from  setting  as  compactly  as  they  would  otherwise 
do. 

If  the  roadway  cannot  be  paved  the  entire  width,  it  should, 
at  least,  receive  a  pavement  for  the  width  of  nine  feet  on  each 
side  of  the  centre.  The  wings,  in  this  case,  may  be  formed 
entirely  of  clean  gravel,  or  of  chippings  of  stone. 

For  roads  which  are  not  much  travelled,  like  the  ordinary  cross 
roads  of  the  country,  the  pavement  will  not  demand  so  much 
care  ;  but  may  be  made  of  any  stone  at  hand,  broken  into  frag- 
ments of  such  dimensions  that  no  stone  shall  weigh  over  four 
pounds.  The  surface-coating  may  be  formed  in  the  manner  just 
described. 

657.  In  forming  a  road-covering  of  broken  stone  alone,  the 
bed  for  the  covering  is  arranged  in  the  same  manner  as  for  the 
paved  bottoming  :  a  layer  of  the  stone,  four  inches  in  thickness, 
is  carefully  spread  over  the  bed,  and  the  road  is  thrown  open  to 
vehicles,  care  being  taken  to  fill  the  ruts,  and  preserve  the  sur- 
face in  a  uniform  state  until  the  layer  has  become  compact; 
successive  layers  are  laid  on  and  treated  in  the  same  manner  as 
the  first,  until  the  covering  has  received  a  thickness  of  about 
twelve  inches  in  the  centn:,  with  the  ordinary  convexity  at  the 
surface. 

658.  Where  good  gravel  can  be  procured  the  road-covering 
may  be  made  of  this  material,  which  should  be  well  screened, 
and  all  pebbles  found  in  it  over  two  and  a  half  inches  in  diame- 
ter should  be  broken  into  fragments  of  not  greater  dimensions 
than  these.  A  firm  level  form  having  been  prepared,  a  layer  of 
gravel,  four  inches  in  thickness,  is  laid  on,  and,  when  this  has 
become  compact  from  the  travel,  successive  layers  of  about  three 
inches  in  thickness  are  laid  on  and  treated  like  the  first,  until  the 
covering  has  received  a  thickness  of  sixteen  inches  in  the  centre 
and  the  ordinary  convexity. 

659.  As  has  been  already  stated,  the  French  civil  engineer! 

38 


298  ROADS. 

do  not  regard  a  paved  bottoming  as  essentia!  for  broken-stone 
road-coverings,  except  in  cases  of  a  very  heavy  traffic,  or  where 
the  substratum  of  the  road  is  of  a  very  yielding  character 
They  also  give  less  thickness  to  the  road-covering  than  the. 
English  engineers  of  Telford's  school  deem  necessary  ;  allowing 
not  more  than  six  to  eight  inches  to  road-coverings  for  light 
traffic,  and  about  ten  inches  only  for  the  heaviest  traffic. 

660.  If  the  soil  upon  which  the  road-covering  is  to  be  placed 
is  not  dry  and  firm,  they  compress  it  by  rolling,  which  is  done 
by  passing  over  it  several  times  an  iron  cylinder,  about  six  feet 
in  diameter,  and  four  feet  in  length,  the  weight  of  which  can  be 
increased,  by  additional  weights,  from  six  thousand  to  about 
twenty  thousand  pounds.  The  road  material  is  placed  upon  the 
bed,  when  well  compressed  and  levelled,  in  layers  of  about  four 
inches,  each  layer  being  compressed  by  passing  the  cylinder 
several  times  over  it  before  a  new  one  is  laid  on.  If  the  opera- 
tion of  rolling  is  performed  in  dry  weather,  the  layer  of  stone  is 
watered,  and  some  add  a  thin  layer  of  clean  sand,  from  four  to 
eight  tenths  of  an  inch  in  thickness,  over  each  layer  before  it  is 
rolled,  for  the  purpose  of  consolidating  the  surface  of  the  layer, 
by  filling  the  voids  between  the  broken-stone  fragments.  After 
the  surface  has  been  well  consolidated  by  rolling,  the  road  is 
thrown  open  for  travel,  and  all  ruts  and  other  displacement  of 
the  stone  on  the  surface  are  carefully  repaired,  by  adding  fresh 
material,  and  levelling  the  ridges  by  ramming. 

Great  importance  is  attached  by  the  French  engineers  to  the 
use  of  the  iron  cylinder  for  compressing  the  materials  of  a  new 
road,  and  to  minute  attention  to  daily  repairs.  It  is  stated  that 
by  the  use  of  the  cylinder  the  road  is  presented  at  once  in  a 
good  travelling  condition  ;  the  wear  of  the  materials  is  less  than 
by  the  old  method  of  gradually  consolidating  them  by  the  travel; 
the  cost  of  repairs  during  the  first  years  is  diminished  ;  it  gives 
to  the  road-covering  a  more  uniform  thickness,  and  admits  of  its 
being  thinner  than  in  the  usual  method. 

661.  Materials  and  Repairs.  The  materials  for  broken-stone 
roads  should  be  hard  und  durable.  For  the  bottom  layer  a  soft 
stone,  or  a  mixture  of  hard  and  soft  may  be  used,  but  on  the 
surface  none  but  the  hardest  stone  will  withstand  the  action  of 
the  wheels.  The  stone  should  be  carefully  broken  into  frag- 
ments of  nearly  as  cubical  a  form  as  practicable,  and  be  cleansed 
from  dirt  and  of  all  very  small  fragments.  The  broken  stone 
should  be  kept  in  depots  at  convenient  points  along  the  line  of 
the  road  for  repairs. 

Too  great  attention  cannot  be  bestowed  upon  keeping  the 
road-surface  free  from  an  accumulation  of  mud  and  even  of  dust. 
It  should  be  constantly  cleaned  by  scraping  and  sweeping.    The 


ROADS.  299 

repairs  should  be  daily  made  by  adding  fresh  material  ujon  all 
points  where  hollows  or  ruts  commence  to  form.  It  is  recom- 
mended by  some  that  when  fresh  material  is  added,  the  surface 
on  which  it  is  spread  should  be  broken  with  a  pick  to  the  depth 
of  half  an  inch  to  an  inch,  and  the  fresh  material  be  well  settled 
by  ramming,  a  small  quantity  of  clean  sand  being  added  to  make 
the  stone  pack  better.  When  not  daily  repaired  by  persons 
whose  sole  business  it  is  to  keep  the  road  in  good  order,  genera1 
repairs  should  be  made  in  the  months  of  October  and  April, 
by  removing  all  accumulations  of  mud,  cleaning  out  the  side 
channels  and  other  drains,  and  adding  fresh  material  where  re- 
quisite. 

The  importance  of  keeping  the  road-surface  at  all  times  free 
from  an  accumulation  of  mud  and  dust,  and  of  preserving  the 
surface  in  a  uniform  state  of  evenness,  by  the  daily  addition  of 
fresh  material  wherever  the  wear  is  sufficient  to  call  for  it,  can- 
not be  too  strongly  insisted  upon.  Without  this  constant  super- 
vision, the  best  constructed  road  will,  in  a  short  time,  be  unfit 
for  travel,  and  with  it  the  weakest  may  at  all  times  be  kept  in  a 
tolerably  fair  state. 

662.  Cross  dimensions  of  roads.  A  road  thirty  feet  in  width 
is  amply  sufficient  for  the  carriage-way  of  the  most  frequented 
thoroughfares  between  cities.  A  width  of  forty,  or  even  sixty 
feet,  may  be  given  near  cities,  where  the  greater  part  of  the 
transportation  is  effected  by  land.  For  cross  roads,  and  others 
of  minor  importance,  the  width  may  be  reduced  according  to  the 
nature  of  the  case.  The  width  should  be  at  least  sufficient  to 
allow  two  of  the  ordinary  carriages  of  the  country  to  pass  each 
other  with  safety.  In  ail  cases,  it  should  be  borne  in  mind,  that 
any  unnecessary  width  increases  both  the  first  cost  of  construc- 
tion, and  the  expense  of  annual  repairs. 

Very  wide  roads  have,  in  some  cases,  been  used,  the  centre 
part  only  receiving  a  road-covering,  and  the  wings,  termed  sum- 
mer roads,  being  formed  on  the  natural  surface  of  the  subsoil. 
The  object  of  this  system  is  to  relieve  the  road-covering  from 
the  wear  and  tear  occasioned  by  the  lighter  kind  of  vehicles  du- 
ring the  summer,  as  the  wings  present  a  more  pleasant  surface 
for  travelling  in  that  season.  But  little  is  gained  by  this  system 
under  this  point  of  view  ;  and  it  has  the  inconvenience  of  form- 
ing during  the  winter  a  large  quantity  of  mud  which  is  very  in- 
jurious to  the  road-covering. 

There  should  be  at  least  one  foot-path,  from  five  to  six  feet 
wide,  and  not  more  than  nine  inches  higher  than  the  bottom  of 
the  side  channels.  The  surface  of  the  foot-path  should  have  a 
pitch  of  two  inches,  towaHs  the  side  channels,  to  convey  its 
surface  water  into  them.     When  the  natural  scil  is  firm  and 


300  ROADS. 

sandy,  or  gravelly,  its  surface  will  serve  for  the  foot-path  ;  but 
in  other  cases  the  natural  soil  must  be  thrown  out  to  a  depth  of 
six  inches,  and  the  excavation  be  filled  with  fine  clean  gravel. 

To  prevent  the  foot-path  from  being  damaged  by  the  current 
of  water  in  the  side  channels,  its  side  slope,  next  to  the  side 
channel,  must  be  protected  by  a  facing  of  good  sods,  or  of  dry 
stone. 

As  it  is  of  the  first  importance,  in  keeping  the  road-way  in  a 
good  travelling  state,  that  its  surface  should  be  kept  dry,  it  will 
be  necessary  to  remove  from  it,  as  far  as  practicable,  all  objects 
that  might  obstruct  the  action  of  the  wind  and  the  sun  on  its 
surface.  Fences  and  hedges  along  the  road  should  not  be  higher 
than  five  feet ;  and  no  trees  should  be  suffered  to  stand  on  the 
road-side  of  the  side  drains,  for  independently  of  shading  the 
road-way,  their  roots  would  in  time  throw  up  the  road-cov 
ering. 


See  Note  B.,  Avvendix. 


RAILWAYS.  301 


RAILWAYS 

663.  The  great  resistance  offered  to  ihe  force  of  traction  oq 
common  roads,  where  the  traffic  is  of  a  heavy  character,  natu- 
rally suggested  the  idea  of  trying  other  means,  which  would 
afford  a  more  even  and  durable  track  for  the  wheels  than  the 
road-coverings  in  ordinary  use.  Various  methods  have  been  re- 
sorted to,  with  greater  or  less  success,  to  accomplish  this  object : 
in  some  instances  tracks  have  been  formed  of  long  narrow  stone 
blocks  ;  in  others,  heavy  beams  of  timber,  covered  on  the  sur- 
face with  sheet  iron  to  protect  them  from  wear,  have  been  used ; 
and  finally,  both  the  stone  and  wooden  ways  were  replaced  by 
iron  plates  and  bars,  and  that  system  of  road-covering,  now  so 
well  known  as  the  railway,  or  railroad,  has  been  the  result. 

For  these  successive  stages  of  improvement,  by  which,  in  the 
short  period  of  less  than  a  quarter  of  a  century,  so  great  a  revo- 
lution has  been  made  both  in  the  speed  and  the  amount  of  trans- 
portation on  land,  by  means  which  bid  fair  to  supersede  every 
other,  the  civilized  world  is  indebted  to  England,  in  whose  mi- 
ning districts  the  railway  system  first  sprung  up. 

664.  A  railway,  or  railroad,  is  a  track  for  the  wheels  of  ve- 
hicles to  run  on,  which  is  formed  of  iron  bars  placed  in  two 
parallel  lines  and  resting  on  firm  supports. 

665.  Rails.  The  iron  ways  first  laid  down,  and  termed  tram- 
ways, were  made  of  narrow  iron  plates,  cast  in  short  lengths, 
with  an  upright  fianch  on  the  exterior  to  confine  the  wheel  within 
the  track.  The  plates  were  found  to  be  deficient  in  strength, 
and  were  replaced  by  others  to  which  a  vertical  rib  was  added 
under  the  plate.  This  rib  was  of  uniform  breadth,  and  of  the 
shape  of  a  semi-ellipse  in  elevation.  This  form  of  tramway, 
although  superior  in  strength  to  the  first,  was  still  found  not  to 
work  well,  as  the  mud  which  accumulated  between  the  fianch 
and  the  surface  of  the  plate  presented  a  considerable  resistance 
to  the  force  of  traction.  To  obviate  this  defect,  iron  bars  of  a 
semi-elliptical  shape  in  elevation,  which  received  the  name  of 

Fig.  157 — Represents  a  cross  section  a,  of  the  fish-bel- 
lied rail  of  the  Liverpool  and  Manchester  Railway, 
and  the  method  in  which  it  is  secured  to  its  chair 
The  rail  is  formed  with  a  slight  projection  at  bot- 
tom, which  fits  into  a  corresponding  notch  in  the 
side  of  the  chair  b.  An  iron  wedge  c  is  inserted 
into  a  notch  on  the  opposite  side  of  the  chair,  and 
confines  the  rail  in  its  place. 

edge  rails,  were  substituted  for  the  plates  of  the  tramway.     The 
cross  sections  of  these  rais  were  of  the  form  shown  in  Fig.  157, 


302  RAII  WAYS. 

the  top  surface  being  slightly  convex,  and  sufficiently  broad  to 
preserve  the  tire  of  the  wheel  from  wearing  unevenly.  This 
change  in  the  form  of  the  rail  introduced  a  corresponding  one  ir 
the  tires  of  the  wheels,  which  were  made  with  a  flanch  on  tht 
interior  to  confine  them  with' n  the  rails  of  the  track. 

The  cast-iron  edge-rail  was  found  upon  trial  to  be  subject  to 
many  defects,  arising  from  the  nature  of  the  material.  As  it 
was  necessary  to  cast  the  rails  in  short  lengths  of  three  or  four 
feet,  the  track  presented  a  number  of  joints,  which  rendered  it 
extremely  difficult  to  preserve  a  uniform  surface.  The  rails 
were  found  to  break  readily,  and  the  surface  upon  which  the 
wheels  ran  wore  unevenly.  These  imperfections  finally  led  to 
the  substitution  of  wrought-iron  for  cast-iron. 

666.  The  wrought-iron  rails  first  brought  into  use  received 
nearly  the  same  shape  in  cross  section  and  elevation  as  the  cast- 
iron  rail.  They  were  formed  by  rolling  them  out  in  a  rolling- 
mill  so  arranged  as  to  give  the  rail  its  proper  shape.  The  length 
of  the  rail  was  usually  fifteen  feet,  the  bottom  of  it  (Fig.  158) 


\   Fifr.  158— Represents  a  side  elevation  of  a  por- 
tion of  a  fish-bellied  rail. 

presenting  an  undulating  outline  so  disposed  as  to  give  the  rail  a 
bearing  point  on  supports  placed  three  feet  apart  between  their 
centres.  This  form,  known  as  the  fish-belly  rail,  was  adopted 
as  presenting  the  greatest  strength  for  the  same  amount  of  metal. 
It  has  been  found  on  trial  to  be  liable  to  some  inconveniences. 
The  rails  break  at  about  nine  inches  from  the  supports,  or  one 
fourth  of  the  distance  between  the  bearing  points,  and  from  the 
curved  form  of  the  bottom  of  the  rail  they  do  not  admit  of  being 
supported  throughout  their  length. 

667.  The  form  of  rail  at  present  in  most  general  use  is 
known  by  the  name  of  the  parallel,  or  straight  rail,  the  top  and 
bottom  of  the  rail  being  parallel ;  or  as  the  T,  or  H  rail,  from  the 
form  of  the  cross  section. 

A  variety  of  forms  of  cross  section  are  to  be  met  with  in  the 
parallel  rail.     The  more  usual  form  is  that  (Fig.  159)  in  which 


Fi§[.  159— Represents  a  cross  sec- 
tion a  of  a  parallel  rail  of  the 
form  generally  adopted  in  the  U. 
States.  The  rail  is  confined  tc 
its  chair  b  by  two  wooden  keys  c 
on  each  side,  which  are  formed 
of  hard  compressed  wood. 


the  top  is  shaped  like  the  same  part  in  the  fish-belly  rail,  the 
oottom  being  widened  out  to  give  the  rail  a  more  stable  seat  on 


RA  LWAYS.  30a 

its  supports.  In  some  cases  the  top  and  bottom  are  made  alike 
to  admit  of  turning  the  rail.  The  greatest  deviation  from  the 
usual  form  is  in  the  rail  of  the  Great  Western  Railway  in  Eng 
land,  (Fig.  160.) 

Fig.  160 — Represents  a  cross  section  of  the  rail  of  the 
Great  Western  Railway  in  England.  This  rail  is  laid 
on  a  continuous  support,  and  is  fastened  to  it  by  screws 
on  each  side  of  the  rail.  A  piece  of  tarred  felt  was 
inserted  between  the  base  of  the  rail  and  its  support. 

The  dimensions  of  the  cross  section  of  a  rail  should  be  such 
that  the  deflection  in  the  centre  between  any  two  points  of  sup- 
port, caused  by  the  heaviest  loads  upon  the  track,  should  not  be 
so  great  as  to  cause  any  very  appreciable  increase  of  resistance 
to  the  force  of  traction.  The  greatest  deflection,  as  laid  down 
by  some  writers,  should  not  exceed  three  hundredths  of  an  inch, 
for  the  usual  bearing  of  three  feet  between  the  points  of  sup- 
port. The  top  of  the  rail  is  usually  about  two  and  a  half  inches 
broad,  and  an  inch  in  depth.  This  has  been  found  to  present  a 
good  bearing  surface  for  the  wheels,  and  sufficient  strength  to 
prevent  the  top  from  being  crushed  by  the  weight  upon  the  rail. 
The  breadth  of  the  rib  varies  between  three  fourths  of  an  inch  to 
an  inch  ;  and  the  total  depth  of  the  rail  from  three  to  five  inches. 
The  thickness  and  breadth  of  the  bottom  have  been  varied  ac- 
cording to  the  strength  and  stability  demanded  by  the  traffic. 

668.  Supports.  The  rails  are  laid  upon  supports  of  timber, 
or  stone.  The  supports  should  present  a  firm  unyielding  bed  to 
the  rails,  so  as  to  prevent  all  displacement,  either  in  a  lateral  or 
a  vertical  direction,  from  the  pressure  thrown  upon  them. 

Considerable  diversity  is  to  be  met  with  in  the  practice  of 
engineers  on  this  point.  On  the  earlier  roads,  heavy  stone 
blocks  were  mostly  used  for  supports,  but  these  were  found  to 
require  great  precautions  to  render  them  firm,  and  they  were, 
moreover,  liable  to  split  from  the  means  taken  to  confine  the 
rails  to  them.  Timber  has,  within  late  years,  been  generally 
preferred  to  stone.  It  affords  a  more  agreeable  road  for  travel, 
and  gives  a  better  lateral  support  to  the  rails  than  stone  blocks. 

The  usual  method  of  placing  timber  supports  is  transversely 
to  the  track.  Each  support,  termed  a  sleeper,  or  cross-tie,  beinp 
formed  of  a  piece  of  timber  six  or  eight  inches  square.  The  or- 
dinary distance  between  the  centre  lines  of  the  supports,  is  three 
feet  for  rails  of  the  usual  dimensions.  With  a  greater  bearing, 
rails  of  the  ordinary  dimensions  do  not  present  sufficient  stiffness. 
The  sleepers,  when  formed  of  round  timber,  should  be  squared 
on  the  upper  and  lower  surface.  On  some  of  the  recent  railwayi 
in  England,  sleepers  presenting  in  c  -oss  section  a  right-angled 
triangle  have  been  used,  the  right  angle  being  at  the  bottom. 
They  ire  represented  to  be  more  convenient  in  setting,  and  tc 


304  RAILWAYS. 

offer  a  more  stable  support  than  those  of  the  usual  fc  rm.  The 
sleepers  are  placed  either  upon  the  ballasting  of  the  roadway,  o* 
upon  longitudinal  beams  laid  beneath  them  along  the  line  of  the 
rails.  The  latter  is  now  the  more  usual  practice  with  us,  and  is 
indispensable  upon  new  embankments  to  prevent  the  ends  of 
the  sleepers  from  settling  unequally.  Thick  plank,  about  eight 
inches  broad  and  three  or  four  inches  thick,  is  usually  employed 
for  the  longitudinal  supports  of  the  sleepers. 

On  some  of  the  more  recent  railways  in  England,  the  rails 
have  been  laid  upon  longitudinal  beams,  presenting  a  continuous 
support  to  the  rail,  the  beams  resting  upon  cross-ties. 

669.  Chairs.  The  rails  are  firmly  fastened  to  their  supports  by 
cast-iron  chairs,  (Figs.  157, 159,)  wrought-iron  spikes,  or  screws. 
The  chair  is  cast  in  one  piece,  and  consists  of  a  bottom-plate,  upon 
which  the  rail  rests,  and  two  side  pieces  between  which  the  rail  is 
confined  by  wedges  of  iron,  or  of  wood.  The  chairs  are  fastened 
to  the  supports  by  iron  bolts,  or  wooden  pins.  A  variety  of 
forms  have  been  given  to  the  chairs,  and  different  methods  adopt- 
ed for  confining  the  rail  firmly  within  them.  Iron  wedges  having 
been  found  to  work  loose,  wooden  wedges,  or  keys,  have  been 
substituted  for  them.  They  are  made  of  kiln-dried  timber,  and 
are  forced  through  cutters,  by  which  they  receive  the  proper 
shape,  and  are  at  the  same  time  strongly  compressed.  The  key, 
prepared  in  this  manner,  gradually  swells  by  imbibing  moisture 
after  being  inserted,  and  forms  a  very  strong  fastening.  Chairs 
are  generally  placed  upon  each  support.  In  some  cases  they 
are  only  placed  at  the  points  of  junction  of  the  rails  ;  iron  spikes 
with  a  bent  head  being  driven  into  the  supports,  to  confine  the 
rails  at  the  intermediate  points  between  the  chairs. 

A  joint  of  sufficient  width  is  left  between  the  ends  of  the  rails, 
to  allow  for  the  expansion  of  the  bars.  Various  methods  of 
forming  this  joint  have  been  tried  ;  the  more  usual  forms  are  the 
square  joint,  and  the  oblique  joint. 

670.  Ballast.  A  covering  of  broke*  stone,  of  clean  coarse 
gravel,  or  of  any  other  material  that  will  allow  the  water  to 
drain  off  freely,  is  laid  upon  the  natural  surface  of  the  excavations 
and  embankments,  to  form  a  firm  foundation  for  the  supports. 
This  has  received  the  appellation  of  the  ballast.  Its  thickness 
is  from  nine  to  eighteen  inches.  Open  or  broken-stone  drains 
should  be  placed  beneath  the  ballasting  to  convey  off  the  surface 
water.  The  parts  of  the  ballasting  upon  which  the  supports 
rest  should  be  well  rammed,  or  rolled  ;  and  it  should  be  wel. 

{>acked  beneath  and  around  the  supports.  After  the  rails  are 
aid,  another  layer  of  broken  stone  or  g-avel  should  be  added 
the  surtace  of  which  should  be  slightly  convex  and  about  thre« 
inches  below  the  top  of  the  rails. 


RAILWAYS.  305 

671.  Temporary  railways  of  wood  and  iron.  On  the  first 
introduction  of  railways  into  the  United  States,  the  tracks  were 
formed  of  flat  iron  bars  laid  upon  longitudinal  beams.  The  iron 
bars  were  about  two  and  a  half  inches  in  breadth,  and  from  one 
half  to  three  fourths  of  an  inch  in  thickness,  the  top  surface 
being  slightly  convex.  They  were  placed  on  the  longitudina 
beams,  a  little  back  from  the  inner  edge,  the  side  of  the  beam 
near  the  top  being  bevelled  off,  and  were  fastened  to  the  beam 
by  screws  or  spikes,  which  passed  through  elliptical  holes  with 
a  countersink  to  receive  the  heads  of  the  spikes ;  the  holes  re- 
ceiving this  shape  to  allow  of  the  contraction  and  expansion  of 
the  bar,  without  displacing  the  fastenings.  The  longitudinal 
beatns  were  supported  by  cross  sleepers,  with  which  they  were 
connected  by  wedges  that  confined  the  beams  in  notches  cut 
into  the  sleepers  to  receive  them.  The  longitudinal  beams  were 
usually  about  six  inches  in  breadth,  and  nine  inches  in  depth, 
and  in  as  long  lengths  as  they  could  be  procured.  The  joints 
between  the  bars  were  either  square  or  oblique,  and  a  piece  of 
iron  or  zinc  was  inserted  ;nto  the  beams  at  the  joint,  to  prevent 
the  end  of  the  rail  from  oeing  crushed  into  the  wood  by  the 
wheels. 

In  some  instances  the  bars  were  fastened  to  long  stone  blocks, 
but  this  method  was  soon  abandoned,  as  the  stone  was  rapidly 
destroyed  by  the  action  of  the  wheels;  besides  which,  the- rigid 
nature  of  the  stone  rendered  the  travelling  upon  it  excessively 
disagreeable. 

This  system  of  railway,  whose  chief  recommendation  is  eco- 
nomy in  the  first  cost,  has  gradually  given  place  to  the  solid  rail. 
Besides  the  want  of  durability  of  the  structure,  it  does  not  pos- 
sess sufficient  strength  for  a  heavy  traffic. 

672.  Gauge.  The  distance  between  the  two  lines  of  rails  of 
a  track,  termed  the  gauge,  which  has  been  adopted  for  the  great 
majority  of  the  railways  in  England,  and  also  with  us,  is  4  feet 
8^  inches.  This  gauge  appears  to  have  been  the  result  of 
chance,  and  it  has  been  followed  in  the  great  majority  of  cases 
up  to  the  present  time,  owing  to  the  inconvenience  that  would 
arise  from  the  adoption  of  a  different  gauge  upon  new  lines. 
The  greatest  deviation  yet  made  from  the  established  gauge  is  in 
that  of  the  Great  Western  Railway,  in  which  the  gauge  is  seven 
feet.  Engineers  are  generally  agreed  that  a  wider  gauge  is  de- 
sirable, as  with  it  the  wheels  of  railway  cars  could  be  made  of 
greater  diameter  than  they  now  receive,  and  be  placed  outside 
of  the  cars  instead  of  under  them  as  at  present ;  the  centre  oi 
gravity  of  the  load  might  be  placed  lower,  and  more  steadi 
ness  of  motion  and  greater  security  at  high  velocities  be  at 
Uined. 

39 


806  RAILWAYS 

In  a  double  track  the  distance  between  the  two  tracks  is  gen- 
erally the  same  as  the  gauge ;  and  the  distance  between  the 
outside  rail  of  a  track,  and  the  sides  of  the  excavation,  or  em- 
bankment, is  seldom  made  greater  than  six  feet,  as  this  is  deemed 
sufficient  to  prevent  the  cars  from  going  over  an  embankment 
were  they  to  run  off  the  rails. 

673.  On  all  straight  portions  of  a  track,  the  supports  should 
be  on  a  level  transversely,  and  parallel  to  the  plane  of  the  track 
longitudinally.  The  top  surface  of  the  rail  should  incline  in- 
ward, to  conform  to  the  conical  form  of  the  wheels ;  this  is 
now  usually  effected  by  giving  the  chair  the  requisite  pitch,  or 
by  forming  the  top  surface  with  the  requisite  bevel  for  this  pur- 
pose. 

674.  Curves.  In  the  curved  portions  of  a  track  the  centri- 
fugal force  tends  to  force  the  carriage  towards  the  outside  rail 
of  the  curve.  •  This  action  of  the  centrifugal  force  is  counter- 
acted, to  a  certain  extent,  by  the  conical  form  of  the  wheels, 
which,  by  causing  them  to  run  on  unequal  diameters  so  soon  as 
they  enter  a  curve,  inclines  the  car  inward.  Within  certain 
limits  of  the  radius  of  curvature,  the  amount  of  the  force  by 
which  the  car  is  impelled  towards  the  centre  of  the  curve,  by 
this  change  in  the  diameter  of  the  interior  and  exterior  wheels,  ■ 
will  be  sufficient  to  counteract  the  centrifugal  force  which  urges 
it  outward.  With  wheels  of  the  diameter  and  shape  at  present 
in  general  use,  the  usual  gauge  of  track,  and  play  between  the 
flanch  of  each  wheel  and  the  side  of  the  rail,  the  least  radius  of 
curvature  which  will  prevent  the  flanch  of  the  exterior  wheel 
from  being  brought  into  contact  with  the  side  of  the  rail,  is  found 
to  be  about  600  feet.  To  prevent  actual  contact  and  offer  per- 
fect security,  the  radius  allowed  should  not  be  less  than  1 000  feet, 
when  the  exterior  and  interior  rails  are  on  the  same  level  trans- 
versely. As  on  curves  with  a  smaller  radius  than  1000  feet, 
the  flanch  of  the  wheel  might  be  driven  against  the  rail,  and  the 
car  be  forced  from  the  track,  it  will  be  requisite  to  provide 
against  this  by  raising  the  exterior  rail  higher  than  the  interior, 
so  that  by  thus  placing  the  wheels  on  an  inclined  plane,  the 
component  of  gravity,  opposed  to  the  centrifugal  force,  added  to 
the  force  which  impels  the  car  inward  when  running  on  wheels 
of  unequal  diameter,  may  balance  the  centrifugal  force.  From 
the  above  conditions  of  equilibrium,  the  elevation  which  the  ex- 
terior rail  should  receive  above  the  interior  can  be  readily  cal- 
culated. The  method  more  usually  adopted,  however,  is  to 
neglect  the  effect  of  the  conical  form  of  the  wheel,  in  counter- 
acting the  action  of  the  centrifugal  force  within  certain  limits, 
and  to  give  the  exterior  rail  an  elevation  sufficient  to  prevent  the 
flanch  of  the  wheel  from  being  driven  against  the  side  of  the  rail 


RAILWAYS. 


307 


when  the  car  is  moving  at  the  highest  supposed  velocity  ;  or,  in 
other  words,  to  give  the  inclined  plane  across  the  track,  on  which 
the  wheels  rest,  an  inclination  such  that  the  tendency  of  the 
wheels  to  slide  towards  the  interior  rail  shall  alone  counteract 
the  centrifugal  force. 

675.  Sidings,  tyc.  On  single  lines  of  railways  short  portions 
of  a  track,  termed  sidings,  are  placed  at  convenient  intervals 
along  the  main  track,  to  enable  cars  going  in  opposite  directions 
to  cross  each  other,  one  train  passing  into  the  sidfng  and  stop- 

{)ing  while  the  other  proceeds  on  the  main  track.  On  double 
ines  arrangements,  termed  crossings,  are  made  to  enable  trains 
to  pass  from  one  track  into  the  other,  as  circumstances  may  re- 
quire. The  position  of  sidings  and  their  length  will  depend 
entirely  on  local  circumstances,  as  the  length  of  the  trains,  the 
number  daily,  &c. 

The  manner  generally  adopted,  of  connecting  the  main  track 
with  a  siding,  or  a  crossing,  is  very  simple.  It  consists  (Fig. 
161)  in  having  two  short  lengths  of  the  opposite  rails  of  the  main 

Fig.  161 — Represents 
the  sliding  switches, 
or  rails,  for  connect- 
ing asiding  with  the 
main  track. 

a,  a,  rails  connected 
by  an  iron  rod  b,  by 
which  they  can  be 
turned  around  the 
joints  of  o. 

c,  c,  rails  of  main 
track. 

d,  d,  rails  of  siding. 

track,  where  the  siding  or  crossing  joins  it,  moveable  around  one 
of  their  ends,  so  that  the  other  can  be  displaced  from  the  line  of 
the  main  track,  and  be  joined  with  that  of  the  siding,  or  crossing, 
on  the  passage  of  a  car  out  of  the  main  track.  These  moveable 
portions  of  rails  are  connected  and  kept  parallel  by  a  long  cross 

M 


\    9 

-3 

— — 

— =T          '\- 

*=—oa 

o 

O 

o      ; 
o 

5 

k S_ 

O 

T    B= 

o    a 

0 

O 

— °       "~" 

\T 


Fig.  162 — Represents  a  plan  M,  and  section  N,  of  a  fixed  crossing  plate  The  plate  A 
is  of  cast-iron,  with  vertical  ribs  c,  c,  on  the  bottom,  to  give  it  the  requisite  strength. 
Wrought-iron  bars  a,  a,  placed  in  the  lines  of  the  two  intersecting  raLb  d,  d,  are 
firmly  screwvd  to  the  plate ;  a  sufficient  space  being  left  between  them  and  the  raik 
"or  the  flanch  of  the  wheel  to  Dass. 


308  RAILWAYS. 

bolt,  to  the  end  of  which  a  vertical  lever  is  attached  to  draw 
them  forward,  or  shove  them  back. 

At  the  point  where  the  rails  of  the  two  tracks  intersect,  a  cast 
iron  plate,  termed  a  crossing-plate  (Fig.  162)  is  placed  to  con 
nect  the  rails.  The  surface  of  the  plate  is  arranged  either  with 
grooves  in  the  lines  of  the  rails  to  admit  the  flanch  of  the  wheel 
in  passing,  the  tire  running  upon  the  surface  of  the  plate ;  or 
wrought-iron  bars  are  affixed  to  the  surface  of  the  plate  for  the 
same  purpose. 

The  angle  between  the  rails  of  the  main  tracks  and  those  of  a 
siding  or  crossing,  termed  the  angle  of  deflection,  should  not  be 
greater  than  2°  or  3°.  The  connecting  rails  between  the  straight 
portions  of  the  tracks  should  be  of  the  shape  of  an  S  curve,  in 
order  that  the  passage  may  be  gradually  effected. 

676.  Turn-plates.  Where  one  track  intersects  another  under 
a  considerable  angle,  it  will  be  necessary  to  substitute  for  the 
ordinary  method  of  connecting  them,  what  is  termed  a  turn-plate, 
or  turn-table.  This  consists  of  a  strong  circular  platform  of 
wood  or  cast-iron,  moveable  around  its  centre  by  means  of  coni- 
cal rollers  beneath  it  running  upon  iron  roller-ways.  Two  rails 
are  laid  upon  the  platform  to  receive  the  car,  which  is  transferred 
from  one  track  to  the  other  by  turning  the  platform  sufficiently 
to  place  the  rails  upon  it  in  the  same  line  as  those  of  the  track 
to  be  passed  into. 

677.  Street-crossings.  When  a  track  intersects  a  road,  or 
street,  upon  the  same  level  with  it,  the  rail  must  be  guarded  by 
cast-iron  plates  laid  on  each  side  of  it,  sufficient  space  being  left 
between  them  and  the  rail  for  the  play  of  the  flanch.  The  top 
of  the  plates  should  be  on  a  level  with  the  top  of  the  rail. 
Wherever  it  is  practicable  a  drain  should  be  placed  beneath,  to 
receive  the  mud  and  dust  which,  accumulating  between  the  plates 
and  rail,  might  interfere  with  the  passing  of  the  cars  along  the 
rails. 

678.  Gradients.  From  various  experiments  upon  the  friction 
of  cars  upon  railways,  it  appears  that  the  angle  of  repose  is 
about  2 jo,  but  that  in  descending  gradients  much  sleeper,  the 
velocity  due  to  the  accelerating  force  of  gravity  soon  attains  its 
greatest  limit  and  remains  constant,  from  the  resistance  caused 
bv  the  air. 

The  limit  of  the  velocity  thus  attained  upon  gradients  of  any 
degree,  whether  the  train  descends  by  the  action  of  gravity  alone, 
or  by  the  combined  action  of  the  motive  power  of  the  engine 
and  gravity,  can  be  readily  determined  for  any  given  load.  From 
calculation  and  experiment  it  appears  that  heavy  trains  may  de- 
scend gradients  of  x\%t  without  attaining  a  greater  velocity  than 
about  40  or  50  miles  an  hour,  by  allowing  them  to  run  freely 


.,  MLWAYS.  309 

without  applying  the  brake  to  check  the  speed.  By  the  appli- 
cation of  the  brake,  the  velocity  may  be  kept  within  any  limit 
of  safety  upon  much  steeper  gradients.  The  only  question,  then, 
in  comparing  the  advantages  of  different  gradients,  is  one  of  the 
comparative  cost  between  the  loss  of  power  and  speed,  on  the 
one  hand,  for  ascending  trains  on  steep  gradient,  and  that  of  the 
heavy  excavations,  tunnels,  and  embankments,  on  the  other, 
which  may  be  required  by  lighter  gradients. 

In  distributing  the  gradients  along  a  line,  engineers  are  gener- 
ally agreed  that  it  is  more  advantageous  to  have  steep  gradients 
upon  short  portions  of  the  line,  than  to  overcome  the  same  dif- 
ference of  level  by  gradients  less  steep  upon  longer  develop- 
ments. 

679.  In  steep  gradients,  where  locomotive  power  cannot  be 
employed,  stationary  power  is  used,  the  trains  being  dragged  up, 
or  lowered,  by  ropes  connected  with  a  suitable  mechanism, 
worked  by  stationary  power  placed  at  the  top  of  the  plane. 
The  inclined  planes,  with  stationary  power,  generally  receive  a 
uniform  slope  throughout.  The  portion  of  the  track  at  ihe  top 
and  bottom  of  the  plane,  should  be  level  for  a  sufficient  distance 
back,  to  receive  the  ascending  or  descending  trains.  The  axes 
of  the  level  portions  should,  when  practicable,  be  in  the  same 
vertical  plane  as  that  of  the  axis  of  the  inclined  plane. 

Small  rollers,  or  sheeves,  are  placed  at  suitable  distances  along 
the  axis  of  the  inclined  plane,  upon  which  the  rope  rests. 

Within  a  few  years  back  flexible  bands  of  rolled  hoop-iron 
have  been  substituted  for  ropes  on  some  of  the  inclined  planes 
of  the  United  States,  and  have  been  found  to  work  well,  pre- 
senting more  durability  and  being  less  expensive  than  ropes. 

660.  Tunnels.  The  great  consumption  of  power  by  gravity, 
and  the  necessity  therefore  of  either  employing  additional  power, 
or  of  diminishing  the  load  of  locomotives  in  ascending  steep  gra- 
dients, have  caused  engineers  to  resort  to  excavations  and  em- 
bankments frequently  of  excessive  dimensions,  to  obtain  gradients 
upon  which  the  ordinary  loads  on  a  level  can  be  transported  with 
a  suitable  degree  of  speed.  The  difficulty  and  cost  of  forming 
these  works  become  in  some  cases  so  great,  that  it  is  found 
preferable  to  obtain  the  requisite  gradient  by  carrying  the  road 
under  ground  by  an  excavation  termed  a  tunnel. 

The  choice  between  deep  cutting  and  tunnelling,  will  depend 
upon  the  relative  cost  of  the  two,  and  the  nature  of  the  ground. 
When  the  cost  of  the  two  methods  would  be  about  equal,  and 
the  slopes  of  the  deep  cut  are  not  liable  to  slips,  it  is  usually 
more  advantageous  to  resort  to  deep  cutting  than  to  tunnelling. 
So  much,  however,  will  depend  upon  local  circumstances,  thai 
the  comparative  advantages  of  the  two  methods  can  only  be  de 


310  RAILWAYS. 

cided  upon  understanding^  when  these  are  known  Where  any 
latitude  of  choice  of  locality  is  allowed,  the  natuie  of  the  soil, 
the  length  of  the  tunnel,  that  of  the  deep  cuts  by  which  it  must 
be  approached,  and  also  the  depths  of  the  working  and  air  shafts, 
must  all  be  well  studied  before  any  definitive  location  is  decided 
upon.  In  some  cases  it  may  be  found,  that  a  longer  tunnel  with 
shorter  deep  cuts  will  be  more  advantageous  in  one  position, 
than  a  shorter  tunnel  with  longer  deep  cuts  in  another.  In  oth- 
ers, the  greater  depth  of  working  shafts  may  be  more  than  com- 
pensated by  obtaining  a  safer  soil,  or  a  shorter  tunnel. 

681.  The  operations  in  tunnelling  will  depend  upon  the  nature 
of  the  soil.  The  work  is  commenced  by  setting  out,  in  the  first 
place,  with  great  accuracy  upon  the  surface  of  the  ground,  the 
profile  line  contained  in  the  vertical  plane  of  the  axis  of  the  tun- 
nel. At  suitable  intervals  along  this  line  vertical  pits,  termed 
working  shafts,  are  sunk  to  a  level  with  the  top,  or  crown  of  the 
tunnel.  The  shafts  and  the  excavations,  which  form  the  en- 
trances to  the  tunnel,  are  connected,  when  the  soil  will  admit  of 
it,  by  a  small  excavation  termed  a  heading,  or  drift,  usually  five 
or  six  feet  in  width,  and  seven  or  eight  feet  in  height,  which  is 
made  along  the  crown  of  the  tunnel.  After  the  drift  is  com- 
pleted, the  excavation  for  the  tunnel  is  gradually  enlarged ;  the 
excavated  earth  is  raised  through  the  working  shafts,  and  at  the 
same  time  carried  out  at  the  ends.  The  dimensions  and  form 
of  the  cross  section  of  the  excavation,  will  depend  upon  the  na- 
ture of  the  soil,  and  the  object  of  the  tunnel  as  a  communi- 
cation. In  solid  rock  the  sides  of  the  excavation  are  usually 
vertical ;  the  top  receives  an  arched  form ;  and  the  bottom  is 
horizontal.  In  soils  which  require  to  be  sustained  by  an  arch, 
the  excavation  should  conform  as  nearly  as  practicable  to  the 
form  of  cross  section  of  the  arch. 

In  tunnels  through  unstratified  rocks,  the  sides  and  roof  may 
be  safely  left  unsupported ;  but  in  stratified  rocks  there  is  dan- 
ger of  blocks  becoming  detached  and  falling :  wherever  this  is 
to  be  apprehended,  the  top  of  the  tunnel  should  be  supported  by 
an  arch. 

Tunnelling  in  loose  soils  is  one  of  the  most  hazardous  opera- 
tions of  the  miner's  art,  requiring  the  greatest  precautions  in 
supporting  the  sides  of  the  excavations  by  strong  rough  frame- 
work, covered  by  a  sheathing  of  boards,  to  secure  the  workmen 
from  danger.  When  in  such  cases  the  drift  cannot  be  extended 
throughout  the  line  of  the  tunnel,  the  excavation  is  advanced 
only  a  few  feet  in  each  direction  from  the  bottom  of  the  working 
shafts,  and  is  gradually  widened  and  deepened  to  the  proper 
form  and  dimensions  to  receive  the  masonry  of  the  tunnel,  which 
is  immediately  commenced  below  each  working  shaft,  and  it 


RAILWAYS.  311 

carried  forward  in  both  directions  towards  the  two  ends  of  thf. 
tunnel. 

682.  Masonry  of  tunnels.     The  cross  section  of  the  arch  of  s 
tunnel  (Fig.  163)  is  usually  an  oval  segment,  formed  of  arcs  of 


Fig.  163— Represents  the  general  form  of 

the  cross  section  c  of  a  brick  arch  for 

tunnels. 
a,  a,  askew-back  stone  between  the  sides 

ot  the  arch  and  the  bottom  inverted 

arch. 


circles  for  the  sides  and  top,  resting  on  an  inverted  arch  at  hot 
torn.  The  tunnels  on  some  of  the  recent  railways  in  England 
are  from  24  to  30  feet  wide,  and  of  the  same  height  from  the 
level  of  the  rails  to  the  crown  of  the  arch.  The  usual  thickness 
of  the  arch  is  eighteen  inches.  Brick  laid  in  hydraulic  cement 
is  generally  used  for  the  masonry,  an  askew  back  course  of  stone 
being  placed  at  the  junction  of  the  sides  and  the  inverted  arch. 
The  masonry  is  constructed  in  short  lengths  of  about  twenty 
feet,  depending,  however,  upon  the  precautions  necessary  to  se- 
cure the  sides  of  the  excavation.  As  the  sides  of  the  arch  are 
carried  up,  the  frame-work  supporting  the  earth  behind  is  grad- 
ually removed,  and  the  space  between  the  back  of  the  ma- 
sonry and  the  sides  of  the  excavation  is  filled  in  with  earth 
well  rammed.  This  operation  should  be  carefulby  attended  to 
throughout  the  whole  of  the  backing  of  the  arch,  so  that  the 
masonry  may  not  be  exposed  to  the  effects  of  any  sudden  yield 
ing  of  the  earth  around  it. 

683.  The  frame- work  of  the  centres  should  be  so  arrangec 
that  they  may  be  taken  apart  and  be  set  up  with  facility.  The 
combination  adopted  will  depend  upon  the  size  of  the  arch,  and 
the  necessity  of  supporting  the  sides  as  well  as  the  top  of  the 
arch  by  the  centre,  during  the  process  of  the  work. 

684.  The  earth  at  the  ends  of  the  tunnel  is  supported  by  a 
retaining  wall,  usually  faced  with  stone.  These  walls,  termed 
the  fronts  of  the  tunnel,  ire  generally  finished  with  the  usua' 


312  RAILWAYS. 

architectural  designs  for  gateways.  To  secure  the  ends  of  the 
arch  from  the  pressure  of  the  earth  above  them,  cast-iron  plates, 
of  the  same  shape  and  depth  as  the  top  of  the  arch,  are  inserted 
within  the  masonry,  a  short  distance  from  the  ends,  and  are  se 
cured  by  wrought-iron  rods  firmly  anchored  to  the  masonry  at 
some  distance  from  each  end. 

685.  The  working  shafts,  which  are  generally  made  cylindri- 
cal and  faced  with  brick,  rest  upon  strong  curbs  of  cast-iron, 
inserted  into  the  masonry  of  the  arch.  The  diameter  of  the  shaft 
within  is  ordinarily  nine  feet. 

Small  shafts,  about  three  feet  in  diameter,  termed  air  shafts, 
are  in  some  cases  required  at  intermediate  points  between  the 
working  shafts,  for  the  purposes  of  ventilation. 

686.  The  ordinary  difficulties  of  tunnelling  are  greatly  increased 
by  the  presence  of  water  in  the  soil  through  which  the  work  is 
driven.  Pumps,  or  other  suitable  machinery  for  raising  water, 
placed  in  the  working  shafts,  will  in  some  cases  be  requisite  to 
keep  them  and  the  drift  free  from  water  until  an  outlet  can  be 
obtained  for  it  at  the  ends,  by  a  drain  along  the  bottom  of  the 
drift.  Sometimes,  when  the  water  is  found  to  gain  upon  the 
pumps  at  some  distance  above  the  level  of  the  crown  of  the 
tunnel,  an  outlet  may  be  obtained  for  it  by  driving  above  the 
tunnel  a  drift-way  between  the  shafts,  giving  it  a  suitable  slope 
from  the  centre  to  the  two  extremities  to  convey  the  water  off 
rapidly. 

In  tunnels  for  railways,  a  drain  should  be  laid  under  the  bal- 
asting  along  the  axis,  upon  the  inverted  arch  of  the  bottom. 


CANALS. 


:m 


CANALS. 

687.  Canals  die  aiiificial  channels  for  water,  applied  to  the 
purpose  of  inland  navigation ;  for  the  supply  of  cities  with  wa- 
ter ;  for  draining ;  for  irrigation,  &c.  &c. 

688.  Navigable  canals  are  divided  into  two  classes  :  1st.  Ca- 
nals which  are  on  the  same  level  throughout  their  entire  length, 
as  those  which  are  found  in  low  level  countries.  2d.  Canals 
which  connect  two  points  of  different  levels,  which  lie  either  in 
the  same  valley,  or  on  opposite  sides  of  a  dividing  ridge.  This 
class  is  found  in  broken  countries,  in  which  it  is  necessary  to 
divide  the  entire  length  of  the  canal  into  several  level  portions, 
the  communication  between  which  is  effected  by  some  artificial 
means.  When  the  points  to  be  connected  lie  on  opposite  sides 
of  a  dividing  ridge,  the  highest  reach,  which  crosses  the  ridge, 
is  termed  the  summit  level. 

689.  1st  Class.  The  surveying  and  laying  out  a  canal  in  a 
level  country,  are  operations  of  such  extreme  simplicity  as  to 
require  no  particular  notice  in  this  place  ;  since  these  operations 
have  been  fully  explained  in  the  subject  of  Common  Roads. 
The  line  of  the  canal  should  be  run  in  a  direct  line  between  the 
two  points  to  be  connected,  unless  it  be  found  necessary  to  de- 
flect it  at  any  intermediate  points  ;  in  which  case,  the  straight 
portions  will  be  connected  by  arcs  of  circles  of  sufficient  curva- 
ture to  allow  the  boats  used  in  the  navigation  to  pass  each  other 
at  the  curves,  without  any  diminution  of  their  ordinary  rate  of 
speed. 

The  cross  section  of  this  class  (Fig.  164)  presents  usually  a 


Fig.  164 — Cross  section  of  a  canal  in  level  cutting. 

A,  water-way. 

B,  tow-paths. 

C,  berms. 

D,  side-drains. 

E,  puddling  of  clay  or  sand 

water-way,  or  channel  of  a  trapezoidal  form,  with  an  embank- 
ment on  each  side,  raised  above  the  general  level  of  the  country, 
and  formed  of  the  excavation  for  the  water-way.     The  level,  of 

40 


814  CANALS. 

surface  of  the  water,  is  usually  above  the  natural  surface,  suffi- 
cient thickness  being  given  to  the  embankments  to  prevent  the 
filtration  of  the  water  through  them,  and  to  resist  its  pressure. 
This  arrangement  has  in  its  favor  the  advantage  of  economy  in 
the  labor  of  excavating  and  embanking,  since  the  cross  section 
of  the  cutting  may  be  so  calculated  as  to  furnish  the  necessary 
earth  for  the  embankment ;  but  it  exposes  the  surrounding  coun- 
try to  injury,  from  accidents  happening  to  the  embankments. 

The  relative  dimensions  of  the  parts  of  the  cross  section  may 
be  generally  stated  as  follows ;  subject  to  such  modifications  as 
each  particular  case  may  seem  to  demand. 

The  width  of  the  water-way,  at  bottom,  should  be  at  least 
twice  the  width  of  the  boats  used  in  navigating  the  canal ;  so 
that  two  boats,  in  passing  each  other,  may,  by  sheering  towards 
the  sides,  avoid  being  brought  into  contact. 

The  depth  of  the  water-way  should  be  at  least  eighteen 
inches  greater  than  the  draft  of  the  boat,  to  facilitate  the  motion 
of  the  boat,  particularly  if  there  are  water-plants  growing  on 
the  bottom. 

The  side  slopes  of  the  water-way,  in  compact  soils,  should 
receive  a  base  at  least  once-and-a-half  the  altitude,  and  propor- 
tionally more  as  the  soil  is  less  compact. 

The  thickness  of  the  embankments,  at  top,  is  seldom  regu 
lated  by  the  pressure  of  the  water  against  them,  as  this,  in  most 
cases,  is  inconsiderable,  but  to  prevent  filtration,  which,  were  it 
to  take  place,  would  soon  cause  their  destruction.  A  thickness 
from  four  to  six  feet,  at  top,  with  the  additional  thickness  given 
by  the  side  slopes  at  the  water  surface,  will,  in  most  cases,  be 
amply  sufficient  to  prevent  nitrations.  A  pathway  for  the  horses 
attached  to  the  boats,  termed  a  tow-path,  which  is  made  on  one 
of  the  embankments,  and  a  foot-path  on  the  other,  which  should 
be  wide  enough  to  serve  as  an  occasional  tow-path,  give  a  su- 
perabundance of  strength  to  the  embankments. 

The  tow-path  should  be  from  ten  to  twelve  feet  wide,  to  allow 
the  horses  to  pass  each  other  with  ease  ;  and  the  foot-patli  at 
least  six  feet  wide.  The  height  of  the  surfaces  of  these  paths, 
above  the  water  surface,  should  not  be  less  than  two  feet,  to 
avoid  the  wash  of  the  ripple;  nor  greater  than  four  feet  and  a 
half,  for  the  facility  of  the  draft  of  the  horses  in  towing.  The 
surface  of  the  tow-path  should  incline  slightly  outward,  both 
to  convey  off  the  surface  water  in  wet  weather,  and  to  give 
a  firmer  footing  to  the  horses,  which  naturally  draw  from  the 
canal. 

The  side  slopes  of  the  embankment  vary  with  the  charactei 
of  the  soil  :  towards  the  water-way  they  should  seldom  be  less 
than  two  base  to  one  perpendicular ;  from  it,  they  may,  if  it  be 


CANALS.  815 

thought  necessary,  be  less.  The  interior  slope  is  usually  not 
carried  up  unbroken  from  the  bottom  to  the  top ;  but  a  horizon- 
tal space,  termed  a  bench,  or  berm,  about  one  or  two  feet  wide, 
is  left,  about  one  foot  above  the  water  surface,  between  the  side 
slope  of  the  water-way  and  the  foot  of  the  embankment  above 
me  berm.  This  space  serves  to  protect  the  upper  part  of  the 
interior  side  slope,  and  is,  in  some  cases,  planted  with  such 
shrubbery  as  grows  most  luxuriantly  in  aquatic  localities,  to  pro- 
tect more  efficaciously  the  banks  by  the  support  which  its  roots 
give  to  the  soil.  The  side  slopes  are  better  protected  by  a  re- 
vetement  of  dry  stone.  Aquatic  plants  of  the  bulrush  kind 
have  been  used,  with  success,  for  the  same  purpose ;  being 
planted  on  the  bottom,  at  the  foot  of  the  side  slope,  they  serve 
to  break  the  ripple,  and  preserve  the  slopes  from  its  effects. 

The  earth  of  which  the  embankments  are  formed  should  be 
of  a  good  binding  character,  and  perfectly  free  from  vegetable 
mould,  and  all  vegetable  matter,  as  the  roots  of  plants,  &c.  In 
forming  the  embankments,  the  vegetable  mould  should  be  care- 
fully removed  from  the  surface  on  which  they  are  to  rest;  and 
they  should  be  carried  up  in  uniform  layers,  from  nine  to  twelve 
inches  thick,  and  be  well  rammed.  If  the  character  of  the  earth, 
of  which  the  embankments  are  formed,  is  such  as  not  to  present 
entire  security  against  filtration,  a  puddling  of  clay,  or  fine  sand, 
two  or  three  feet  thick,  may  be  laid  in  the  interior  of  the  mass, 
penetrating  a  foot  below  the  natural  surface.  Sand  is  useful  in 
preventing  filtration  caused  by  the  holes  made  in  the  embank- 
ments near  the  water  surface  by  insects,  moles,  rats,  &c. 

Side  drains  must  be  made,  on  each  side,  a  foot  or  two  from 
the  embankments,  to  prevent  the  surface  water  of  the  natural 
surface,  from  injuring  the  embankments. 

G90.  2d  Class.  This  class  will  admit  of  two  subdivisions  : 
1st,  Canals  which  lie  throughout  in  the  same  valley ;  2d,  Canals 
with  a  summit  level. 

Location.  In  laying  out  canals,  belonging  to  the  first  sub- 
division, the  line  of  direction  of  the  canal  should  be  as  direct  as 
practicable  between  the  two  points.  As  the  different  levels, 
however,  must  be  laid  out  on  one  of  the  side  slopes  of  the  val- 
ley, their  lines  of  direction  will  be  nearly  the  same  as  the  hori- 
zontal curved  line  in  which  the  natural  surface  of  the  ground 
would  be  intersected  by  the  water  surface  of  the  canal  pro- 
duced ;  the  variations  in  direction  from  this  curve  depending  on 
the  character  of  the  cuttings  and  fillings,  both  as  to  the  advan- 
tages which  the  one  may  present  over  the  other  as  regards  filtra- 
tion, and  the  economy  of  construction. 

With  respect  to  the  side  slope  of  the  valley  along  which  the 
canal  is  to  be  run,  the  engineer  must  be  guided  in  his  choice  by 


316  CANALS. 

the  relative  expense  of  construction  on  the  two  sides ;  which 
will  depend  on  the  quantity  of  cutting  and  filling,  the  masonry 
for  the  culverts,  &c,  and  the  nature  of  the  soil  as  adapted  tc 
holding  water.  All  other  things  being  equal,  the  side  on  which 
the  fewest  secondary  water-courses  are  found  will,  generally 
speaking,  offer  the  greatest  advantage  as  to  expense  ;  but,  it  may 
happen  that  the  secondary  water-courses  will  be  required  to  feecl 
the  canal  with  water,  in  which  case  it  will  be  necessary  to  lay 
out  the  line  on  the  side  where  they  are  found  most  convenient, 
and  in  most  abundance. 

As  to  the  points  in  which  the  line  of  direction  should  cross  the 
secondary  valleys,  the  engineer  will  be  guided  by  the  same  con- 
siderations as  for  any  other  line  of  communication ;  crossing 
them  by  following  the  natural  surface,  or  else  by  a  filling  in  a 
right  line,  as  may  be  most  economical. 

691.  Cross  section.  The  side  formations  of  excavations  and 
embankments  require  peculiar  care,  particularly  the  latter,  as 
any  crevices,  when  they  are  first  formed,  or  which  may  take 
place  by  settling,  might  prove  destructive  to  the  work.  In  most 
cases,  a  stratum  of  good  binding  earth,  lining  the  water-way 
throughout  to  the  thickness  of  about  four  feet,  if  compactly 
rammed,  will  be  found  to  offer  sufficient  security,  if  the  sub- 
structure is  of  a  firm  character,  and  not  liable  to  settle.  Fine 
sand  has  been  applied  with  success  to  stop  the  leakage  in  canals. 
The  sand  for  this  purpose  is  sprinkled,  in  small  quantities  at  a 
time,  over  the  surface  of  the  water,  and  gradually  fills  up  the 
outlets  in  the  bottom  and  sides  of  the  canal.  But  neither  this 
nor  puddling  has  been  found  to  answer  in  all  cases,  particularly 
where  the  substructure  is  formed  of  fragments  of  rocks  offering 
large  crevices  to  filtrations,  or  is  of  a  marly  nature.  In  such 
cases  it  has  been  found  necessary  to  line  the  water-way  through- 
out with  stone,  laid  in  hydraulic  mortar.  A  lining  of  this  cha- 
racter, (Fig.  165,)  both  at  the  bottom  and  sides,  formed  of  flat 


Fig.  165 — Cross  section  of  a  canal  in  side  cutting  ined  with 

A,  water-way. 

B,  tow-paths. 

D,  embankment. 
a,  masonry  lining. 


CANALS.  311 

atones,  about  four  inches  thick,  laid  on  a  bed  of  hydraulic  mor- 
tar, one  inch  thick,  and  covered  by  a  similar  coat  of  mortar, 
making  the  entire  thickness  of  the  lining  six  inches,  has  been 
found  to  answer  all  the  required  purposes.  This  lining  should 
be  covered,  both  at  bottom  and  on  the  sides,  by  a  layer  of  good 
earth,  at  least  three  feet  thick,  to  protect  it  from  the  shock  of  the 
boats  striking  either  of  those  parts. 

The  cross  section  of  the  canal  and  its  tow-paths  in  deep  cut- 
ting (Fig.  166)  should  be  regulated  in  the  same  way  as  in  canals 


Fig.  166 — Cross  section  of  a  canal  in  deep  cutting. 
£,  side  slopes  of  cutting. 

of  the  first  class ;  but  when  the  cuttings  are  of  considerable 
depth,  it  has  been  recommended  to  reduce  both  to  the  dimen- 
sions strictly  necessary  for  the  passage  of  a  single  boat.  By 
this  reduction  there  would  be  some  economy  in  the  excavations ; 
but  this  advantage  would,  generally,  be  of  too  trifling  a  charac- 
ter to  be  placed  as  an  offset  to  the  inconveniences  resulting  to 
the  navigation,  particularly  where  an  active  trade  was  to  be  car- 
ried on. 

692.  Summit  level.  As  the  water  for  the  supply  of  the  sum- 
mit level  of  a  canal  must  be  collected  from  the  ground  that  lies 
above  it,  the  position  selected  for  the  summit  level  should  be  at 
the  lowest  point  practicable  of  the  dividing  ridge,  between  the 
two  branches  of  the  canal.  In  selecting  this  point,  and  the  di- 
rection of  the  two  branches  of  the  canal,  the  engineer  will  be 
guided  by  the  considerations  with  regard  to  the  natural  features 
of  the  surface,  which  have  already  been  dwelt  upon. 

693.  Supply  of  water.     The  quantity  of  water  required  fo~ 
canals  with  a  summit  level,  may  be  divided  into  two  portions 
1st.  That  which  is  required  for  the  summit  level,  and  those  lev 
els  which  draw  from  it  their  supply.     2d.  That  which  is  wanted 
for  the  levels  below  those,  and  which  is  furnished  from  other 
sources. 

The  supply  of  the  first  portion,  which  must  be  collected  at 
the  summit  level,  may  be  divided  into  several  elements :  1st. 
The  quantity  required  to  fill  the  summit  level,  and  the  levels 
which  draw  their  supply  from  it.  2d.  The  quantity  required  to 
supply  losses,  arising  from  accidents ;  as  breaches  in  the  banks, 


318  CANALS. 

and  the  emptying  of  the  levels  for  repairs.  3d.  The  suppliet 
for  losses  from  surface  evaporation,  from  leakage  through  the 
soil,  and  through  the  lock  gates.  4.  The  quantity  required  for 
the  service  of  the  navigation,  arising  from  the  passage  of  the 
boats  from  one  level  to  another.  Owing  to  the  want  of  sufficient 
data,  founded  on  accurate  observations,  no  precise  amount  can 
be  assigned  to  these  various  elements  which  will  serve  the  engi- 
neer as  data  for  rigorous  calculation. 

The  quantity  required,  in  the  first  place,  to  fiil  the  summit 
level  and  its  dependent  levels,  will  depend  on  theii  size,  an  ele- 
ment which  can  be  readily  calculated  ;  and  upon  the  quantity 
which  would  soak  into  the  soil,  which  is  an  element  of  a  very 
indeterminate  character,  depending  on  the  nature  of  the  soil  in 
the  different  levels. 

The  supplies  for  accidental  losses  are  of  a  still  less  determi- 
nate character. 

To  calculate  the  supply  for  losses  from  surface  evaporation, 
correct  observations  must  be  made  on  the  yearly  amount  of 
evaporation,  and  the  quantity  of  rain  that  falls  on  the  surface  ;  as 
the  loss  to  be  supplied  will  be  the  difference  between  these  two 
quantities. 

With  regard  to  the  leakage  through  the  soil,  it  will  depend  on 
the  greater  or  less  capacity  which  the  soil  has  for  holding  water. 
This  element  varies  not  only  with  the  nature  of  the  soil,  but  also 
with  the  shorter  or  longer  time  that  the  canal  may  have  been  in 
use  ;  it  having  been  found  to  decrease  with  time,  and  to  be, 
comparatively,  but  trifling  in  old  canals.  In  ordinary  soils  it 
may  be  estimated  at  about  two  inches  in  depth  every  twenty-four 
hours,  for  some  time  after  the  canal  is  first  opened.  The  leak- 
age through  the  gates  will  depend  on  the  workmanship  of  these 
parts.  From  experiments  by  Mr.  Fisk,  on  the  Chesapeake  and 
Ohio  canal,  the  leakage  through  the  locks  at  the  summit  level, 
which  are  100  feet  long,  15  feet  wide,  and  have  a  lift  of  8  feet, 
amounts  to  twelve  locks  full  daily,  or  about  62  cubic  feet  per 
minute.  The  monthly  loss  upon  the  same  canal,  from  evapora- 
tion and  filtration,  is  about  twice  the  quantity  of  water  contained 
in  it.  From  experiments  made  by  Mr.  J.  B.  Jervis,  on  the  Erie 
(anal,  tne  total  ioss,  trom  evaporation,  filtration,  and  leakage 
.iirough  the  gates,  is  about  100  cubic  feet  per  minute,  for  each 
mile. 

(n  estimating  the  quantity  of  water  expended  for  the  service 
of  the  navigation,  in  passing  the  boats  from  one  level  to  another, 
two  distinct  cases  require  examination  : — 1st.  Where  there  is  but 
one  lock  between  two  levels,  or  in  other  words,  when  the  locks 
are  isolated.  2d.  When  there  are  several  contiguous  locks,  or 
as  it  is  termed,  a  flight  of  locks  between  two  levels. 


:anals.  319 

694.  A  lock  is  a  small  basin  just  large  enough  to  receive  a 
boat,  in  which  the  water  is  usually  confined  on  the  sides  by  two 
upright  walls  of  masonry,  and  at  the  ends  by  two  gates,  which 
open  and  shut,  both  for  the  purpose  of  allowing  the  boat  to  pass, 
and  to  cut  off  the  water  of  the  upper  level  from  the  lower,  as 
well  as  from  the  lock  while  the  boat  is  in  it.  To  pass  a  boat 
from  one  level  to  the  other — from  the  lower  to  the  upper  end, 
for  example — the  lower  gates  are  opened,  and  the  boat  having 
entered  the  lock  they  are  shut,  and  water  is  drawn  from  the  up- 
per level,'  by  means  of  valves,  to  fill  the  lock  and  raise  the  boat , 
when  this  operation  is  finished,  the  upper  gates  are  opened,  and 
the  boat  is  passed  out.  To  descend  from  the  upper  level,  the 
lock  is  first  filled  ;  the  upper  gates  are  then  opened,  and  the  boat 
passed  in  ;  these  gates  are  next  shut,  and  the  water  is  drawn 
from  the  lock,  by  valves,  until  the  boat  is  lowered  to  the  lower 
level,  when  the  lower  gates  are  opened  and  the  boat  is  passed 
out. 

In  the  two  operations  just  described,  it  is  evident,  that  for  the 
passage  of  a  boat,  up  or  down,  a  quantity  of  water  must  be 
drawn  from  the  upper  level  to  fill  the  lock  to  a  height  which  is 
equal  to  the  difference  of  level  between  the  surface  of  the  water 
in  the  two ;  this  height  is  termed  the  lift  of  the  lock,  and  the 
volume  of  water  required  to  pass  a  boat  up  or  down  is  termed 
the  prism  of  lift.  The  calculation,  therefore,  for  the  quantity 
of  water  requisite  for  the  service  of  the  navigation,  will  be  sim- 
ply that  of  the  number  of  prisms  of  lift  which  each  boat  will 
draw  from  the  summit  level  in  passing  up  or  down. 

695.  Let  a  boat,  on  its  way  up,  be  supposed  to  have  arrived 
at  the  lowest  level  supplied  from  the  summit  level ;  it  will  re- 
quire a  prism  of  lift  to  ascend  the  next  level  above,  and  so  on  in 
succession,  until  it  reaches  the  summit  level,  from  which  one 
prism  of  lift  must  be  drawn  to  enable  the  boat  to  enter  it.  From 
this  it  appears  that  but  one  prism  of  lift  is  drawn  from  the  sum- 
mit level  for  the  passage  of  a  boat  up.  Now,  in  descending  on 
the  other  side,  the  boat  will  require  one  prism  of  lift  to  take  it  to 
the  next  level,  and  this  prism  of  lift  will  carry  it  through  all  the 
successive  locks,  if  their  lifts  are  the  same.  For  the  entire  pas- 
sage of  one  boat,  then,  two  prisms  of  lift  must  be  drawn  from 
the  summit  level. 

This  boat  will  thus  leave  all  the  locks  full  on  the  side  of  the 
ascent,  and  empty  on  the  side  of  the  descent.  Now  the  next  boat 
may  be  going  in  the  same,  or  in  an  opposite  direction,  with  re- 
spect to  the  first.  If  it  follows  the  first,  it  will  evidently  require 
two  prisms  of  lift  for  its  entire  passage,  and  will  leave  the  locks 
in  the  same  state  as  they  were.  If  it  proceeds  in  an  opposite 
direction,  i*  will  require  a  prism  of  lift  to  ascend  to  the  summit 


320  CANALS. 

'evel ;  but,  in  descending,  it  will  take  advantage  of  the  full  lock, 
left  by  the  preceding  boat,  and  will  therefore  not  draw  from  the 
summit  level  for  its  descent  to  the  next;  the  same  will  take 
place  at  every  level  until  the  last,  where  it  will  carry  out  with  it 
the  prism  of  lift,  which  was  drawn  from  the  summit  level  for  the 
preceding  boat,  so  that  in  this  case  it  will  draw  but  one  prism 
of  lift  from  the  summit  level.  If  the  two  boats  had  met  on  the 
summit  level,  the  same  would  have  taken  place:  therefore,  when 
the  boats  alternate  regularly,  each  will  require  but  one  prism  of 
lift  for  its  entire  passage.  But  as  this  regularity  of  alternation 
cannot  be  practically  carried  into  effect,  an  allowance  of  two 
prisms  of  lift  must  be  made  for  the  entire  passage  of  each  boat. 

In  calculating  the  expenditure  for  locks  in  flights,  a  new  ele- 
ment, termed  the  pris?n  of  draught,  must  be  taken  into  account. 
This  prism  is  the  quantity  of  water  required  to  float  the  boat  in 
the  lock  when  the  prism  of  lift  is  drawn  off;  and  is  evidently 
equal  in  depth  to  the  water  in  the  canal,  unless  it  should  be 
deemed  advisable  to  make  it  just  sufficient  for  the  draught  of  the 
boat,  by  which  a  small  saving  of  water  might  be  effected. 

696.  Locks  in  flights  may  be  considered  under  two  points  of 
view,  with  regard  to  the  expenditure  of  water :  the  first,  where 
both  the  prism  of  lift,  and  that  of  draught,  are  drawn  off  for  the 
passage  of  a  boat ;  or  second,  where  the  prisms  of  draught  are 
always  retained  in  the  locks.  The  expenditure,  of  course,  will 
be  different  for  the  two  cases. 

To  ascertain  what  will  take  place  in  the  two  cases,  let  a  case 
be  supposed,  in  which  there  is  a  flight  of  locks  on  each  side  of 
the  summit  level,  to  connect  it  w-ith  the  two  next  lower  levels. 
In  the  first  case,  a  boat,  arriving  at  the  foot  of  the  flight,  finds 
all  the  locks  of  the  flight  empty,  except  the  lowest,  which  must 
contain  a  prism  of  draught  to  float  the  boat  in.  To  raise  the 
boat,  then,  to  the  upper  level,  all  the  locks  of  the  flight  must  be 
filled  from  the  summit  level,  which  will  require  as  many  prisms 
of  lift  as  there  are  locks,  and  as  many  prisms  of  draught  as  there 
are  locks  less  one  ;  or,  representing  by  l  the  prism  of  lift,  d  the 
prism  of  draught,  and  n  the  number  of  locks  in  the  flight,  the 
total  quantity  of  water,  for  the  ascent  of  the  boat,  will  be  repre- 
sented by  ,  ,  lX  ,,s 
J            nh  +  (n—  1)  d;     .     .     .     (1). 

In  descending,  on  the  opposite  side,  the  boat  will  require  a  prism 
of  lift  and  one  of  draught  at  the  first  lock  ;  but  to  enter  the  sec- 
ond another  prism  of  draught  in  addition  will  be  required,  anc 
this  entire  quantity  will  be  sufficient  to  take  it  through  ail  tl.< 
remaining  locks  of  the  flight :  this  quantity  will  therefore  i,- »  rtt 
resented  bv 

l  +  2d; (2). 


CANALS.  321 

bo  that  for  the  entire  passage  of  the  boat,  the  total  expenditure 
will  be  represented  by 

(n+"l)L  +  (n  +  l)D.      .     (3). 

The  flight,  on  one  side,  is  thus  left  full  after  the  passage  of 
the  first  boat,  and  on  the  other  side,  empty.  If  a  second  boat, 
then,  follows  directly  after  the  first,  the  prism  of  lift  must  be 
drawn  from  the  lowest  lock  to  admit  the  boat,  this  prism  is  then 
supplied  from  the  lock  next  above,  and  so  on  to  the  summit  lev- 
el ;  so  that  but  one  prism  of  lift  will  be  drawn  off  for  the  ascent 
of  this  boat,  and  it  will  require  one  of  lift,  and  two  of  draught, 
to  carry  it  down  the  opposite  flight.  If,  therefore,  the  total 
number  of  boats  which  follow  in  this  order,  including  the  first, 
be  represented  by  m,  the  total  expenditure  will  be  represent- 
ed by 

(n~+  1)  l  +  (n  +  1)  d  +  (m  —  1)  2l  +  (m  —  1)  2d.    .    (4). 

If  the  second  boat,  instead  of  following  the  first,  arrives  in 
the  opposite  direction,  or  alternates  with  it,  the  expenditure  for 
its  ascent  will  be  represented  by  the  formula  (1),  and  for  its  de- 
scent it  will  be  nothing,  since  it  finds  the  opposite  flight  filled, 
as  left  by  the  first  boat ;  but  if  the  locks  had  been  emptied,  then 
the  passage  of  the  second  boat  would  have  taken  place  under 
the  same  circumstances  as  that  of  the  first. 

It  will  be  unnecessary  here  to  go  farther  into  these  calcula- 
tions for  the  various  cases  that  may  occur,  under  the  different 
circumstances  of  passage  of  the  boats  or  of  empty  or  full  flights  ; 
the  preceding  gives  the  spirit  of  the  method,  and  will  give  the 
means  for  entering  upon  a  calculation  to  allow  for  the  loss  or 
gain  by  the  passage  of  freighted  or  of  empty  boats,  following 
any  prescribed  order  of  passage.  These  refinements  are,  for 
the  most  part,  more  curious  than  useful;  and  the  engineer  should 
confine  himself  to  making  an  ample  allowance  for  the  most  un- 
favorable cases,  both  as  regards  the  order  of  passage  and  the 
number  of  boats. 

697.  Feeders  and  Reservoirs.  Having  ascertained,  from  the 
preceding  considerations,  the  probable  supply  which  should  be 
collected  at  the  summit  level,  the  engineer  will  next  direct  his 
attention  to  the  sources  from  which  it  may  be  procured.  Theo- 
retically considered,  all  the  water  that  drains  from  the  ground 
adjacent  to  the  summit  level,  and  above  it,  might  be  collected  for 
its  supply  ;  but  it  is  found  in  practice  that  channels  for  the  con- 
veyance of  water  must  have  certain  slopes,  and  that  these  slopes, 
moreover,  will  regulate  the  supply  furnished  in  a  certain  time, 
all  other  things  being  equal.  In  making,  however,  the  survey 
of  the  country,  from  which  the  water  is  to  be  supplied  to  the 
tummit  level,  all  the  gruiind  above  it  should  be  examined,  leav- 

41 


322  CANALS. 

ing  the  determination  of  the  slopes  for  after  considerations.  The 
survey  for  this  object  consists  in  making  an  accurate  delineation 
of  all  the  water-courses  above  the  summit  level,  and  in  ascer- 
taining the  quantity  of  water  which  can  be  furnished  by  each  in 
a  given  time.  This  survey,  as  well  as  the  measurement  of  the 
quantity  of  water  furnished  by  each  stream,  which  is  termed  the 
gauging,  should  be  made  in  the  driest  season  of  the  year,  in  or- 
der to  ascertain  the  minimum  supply. 

698.  The  usual  method  of  collecting  the  water  of  the  sources, 
and  conveying  it  to  the  summit  level,  is  by  feeders  and  reser- 
voirs. The  feeder  is  a  canal  of  a  small  cross  section,  which  is 
traced  on  the  surface  of  the  ground  with  a  suitable  slope,  to 
convey  the  water  either  into  the  reservoir,  or  direct  to  the 
summit  level.  The  dimensions  of  the  cross  section,  and  the 
longitudinal  slope  of  the  feeder,  should  bear  certain  relations  to 
each  other,  in  order  that  it  shall  deliver  a  certain  supply  in  a 
given  lime.  The  smaller  the  slope  given  to  the  feeder,  the  lower 
will  be  the  points  at  which  it  will  intersect  the  sources  of  supply, 
and  therefore  the  greater  will  be  the  quantity  of  water  which  it 
will  receive.  This  slope,  however,  has  a  practical  limit,  which 
is  laid  down  at  four  inches  in  1000  yards,  or  nine  thousand  base 
to  one  altitude ;  and  the  greatest  slope  should  not  exceed  that 
which  would  give  the  current  a  greater  mean  velocity  than  thir- 
teen inches  per  second,  in  order  that  the  bed  of  the  feeder  may 
not  be  injured.  Feeders  are  furnished,  like  ordinary  canals, 
with  contrivances  to  let  off  a  part,  or  the  whole,  of  the  water  in 
them,  in  cases  of  heavy  rains,  or  for  making  repairs. 

But  a  small  proportion  of  the  water  collected  by  the  feeders 
is  delivered  at  the  reservoir ;  the  loss  from  various  causes  being 
much  greater  in  them  than  in  canals.  From  observations  made 
on  some  of  the  feeders  of  canals  in  France,  which  have  been  in 
use  for  a  long  period,  it  appears  that  the  feeder  of  the  Briare 
canal  delivers  only  about  one  fourth  of  the  water  it  gathers  from 
its  sources  of  supply ;  and  that  the  annual  loss  of  the  two  feed- 
ers of  the  Languedoc  canal,  amounts  to  100  times  the  quantity 
of  water  which  they  can  contain. 

699.  A  reservoir  is  a  large  pond,  or  body  of  water,  held  in 
resei  ve  for  the  necessary  supply  of  the  summit  level.  A  reser- 
voir is  usually  formed  by  choosing  a  suitable  site  in  a  deep  and 
narrow  valley,  which  lies  above  the  summit  level,  and  erecting  a 
dam  of  earth,  or  of  masonry,  across  the  outlet  of  the  valley,  or 
at  some  more  suitable  point,  to  confine  the  water  to  be  collected. 
The  object  to  be  attained,  in  this  case,  is  to  embody  the  greatest 
volume  of  water,  and  at  the  same  time  present  the  smallest 
evaporating  surface,  at  the  smallest  cost  for  the  construction  of 
the  dam 


CANALS. 


323 


It  is  generally  deemed  best  to  have  two  reservoirs  for  the  sup- 
ply, one  to  contain  the  greater  quantity  of  water,  and  the  other, 
which  is  termed  the  distributing  reservoir,  to  regulate  the  sup- 
ply to  the  summit  level.  If,  however,  the  summit  level  is  very 
capacious,  it  may  be  used  as  the  distributing  reservoir. 

The  proportion  between  the  quantity  of  water  that  falls  upon 
a  given  surface,  and  that  which  can  be  collected  from  it  for  the 
supply  of  a  reservoir,  varies  considerably  with  the  latitude,  the 
season  of  the  year,  and  the  natural  features  of  the  locality.  The 
drainage  is  greatest  in  high  latitudes,  and  in  the  winter  and  spring 
seasons ;  with  respect  to  the  natural  features,  a  wooded  surface 
with  narrow  and  deep  valleys  will  yield  a  larger  amount  than  an 
open  flat  country. 

But  few  observations  have  been  made  on  this  point  by  engi- 
neers. From  some  by  Mr.  J.  B.  Jervis,  in  reference  to  the 
reservoirs  for  the  Chenango  canal,  in  the  state  of  New  York,  it 
appears  that  in  that  locality  about  two  fifths  of  the  quantity  of 
rain  may  be  collected  for  the  supply  of  a  reservoir.  The  pro- 
portion usually  adopted  by  engineers  is  one  third. 

The  loss  of  water  from  the  reservoir  by  evaporation,  filtration, 
and  other  causes,  will  depend  upon  the  nature  of  the  soil,  and 
the  exposure  of  the  water  surface.  From  observations  made 
upon  some  of  the  old  reservoirs  in  England  and  France,  it  ap- 
pears that  the  daily  loss  averages  about  half  an  inch  in  depth. 

700.  The  dams  of  reservoirs  have  been  variously  constructed: 
in  some  cases  they  have  been  made  entirely  of  earth,  (Fig.  167;) 


Fig.  167— Represents  the  section  of  a  dam  with  three  discharging  culverts. 

A,  body  of  the  dam. 

B,  pond. 

a,  a,  a,  culverts,  with  valves  at  their  inlets,  which  discharge  into  the  vertical  well  b. 

c,  c,  c,  grooves,  in  the  faces  of  the  side-walls,  which  form  the  entrance  to  the  culverts, 
for  stop- plank 

d,  stop-plank  dam  across  the  outlet  of  the  bottom  culvert,  to  dam  back  the  water  Late 
the  vertical  well. 

t ,  parapet  wall  on  top  of  the  dam. 

in  others,  entirely  of  masonry ;  and  in  others,  of  earth  packed  in 
between  several  parallel  stone  walls.     It  is  now  thought  best  to 


324  CANALS. 

use  either  earth  or  masonry  alone,  according  to  the  circum 
stances  of  the  case  ;  the  comparative  expense  of  the  two  meth 
ods  being  carefully  considered. 

Earthen  dams  should  :>e  made  with  extreme  care,  of  the  besl 
binding  earth,  well  freed  from  every  thing  that  might  cause  fil- 
trations.  A  wide  trench  should  be  excavated  to  the  firm  soil,  U 
receive  the  base  of  the  dam ;  and  the  earth  should  be  carefully 
spread  and  rammed  in  layers  not  over  a  foot  thick.  As  a  farther 
precaution,  it  has  in  some  instances  been  thought  necessary  to 
place  a  stratum  of  the  best  clay  puddling  in  the  centre  of  the 
dam,  reaching  from  the  top  to  three  or  four  feet  below  the  base. 
The  dam  may  be  from  fifteen  to  twenty  feet  thick  at  top.  The 
slope  of  the  dam  towards  the  pond  should  be  from  three  to  six 
base  to  one  perpendicular ;  the  reverse  slope  need  only  be  some- 
what greater  than  the  natural  slope  of  the  earth. 

The  slope  of  dams  exposed  to  the  water  is  usually  faced 
with  dry  stone,  to  protect  the  dam  from  the  action  of  the  surface 
ripple.  This  kind  of  facing  has  not  been  found  to  withstand 
well  the  action  of  the  water  when  agitated  by  high  winds.  Upon 
some  of  the  more  recent  earthen  dams  erected  in  France,  a  facing 
of  stone  laid  in  hydraulic  mortar  has  been  substituted  for  the  one 
of  dry  stone.  The  plan  adopted  for  this  facing  (Fig.  166)  con- 
Fig.  168 — Represents  the  method 
of  facing  the  pond  slope  of  a 
dam,  with  low  walls  placed  in 
offsets. 
A,  body  of  the  dam. 

a,  a.  a,  low  walls,  the  faces  of 
which  are  built  in  offsets. 

b,  b,  top  surface  of  the  offsets  be- 
tween the  walls,  covered  with 
stone  slabs  laid  in  mortar. 

c,  top  of  dam  faced  like  the  off- 
sets b. 

d,  parapet  wall. 

sists  in  placing  a  series  of  low  walls,  in  offsets  above  each  other, 
along  the  slope  of  the  dam,  covering  the  exposed  surface  of  each 
offset,  between  the  top  of  one  wall  and  the  foot  of  the  next,  with 
a  coating  of  slab-stone  laid  in  mortar.  The  walls  are  from  five 
to  six  feet  high.  They  are  carried  up  in  small  offsets  upon  the 
face,  and  are  made  either  vertical,  or  leaning,  on  the  back.  The 
width  of  the 'offsets  of  the  dam,  between  the  top  of  one  wall  and 
the  foot  of  the  next,  is  from  two  to  three  feet. 

An  arched  culvert,  or  a  large  cast-iron  pipe,  placed  at  some 
suitable  point  of  the  base  of  the  dam,  which  can  be  closed  or 
opened  by  a  valve,  will  serve  for  drawing  off  the  requisite 
supply  of  water,  and  for  draining  the  reservoir  in  case  of  re- 
pairs. 

The  culvert  should  be  strongly  constructed,  and  the  earth 


CANALS.  325 

around  it  be  well  puddled  and  rammed,  to  prevent  filtrations. 
Its  size  should  be  sufficient  for  a  man  to  enter  it  with  ease. 
The  valves  may  be  placed  either  at  the  entrance  of  the  culvert, 
or  at  some  intermediate  point  between  the  two  ends.  Great 
care  should  be  taken  in  their  arrangement,  to  secure  them  from 
accidents. 

When  the  depth  «of  water  in  a  reservoir  is  considerable,  several 
culverts  should  be  constructed,  (Fig.  167,)  to  draw  off  the  water  at 
different  levels,  as  the  pressure  upon  the  lower  valves  in  this  case 
would  be  very  great  when  the  reservoir  is  full.  They  may  be 
placed  at  intervals  of  about  twelve  feet  above  each  other,  and  be 
arranged  to  discharge  their  water  in  a  common  vertical  shaft. 
In  this  case  it  will  be  well  to  place  a  dam  of  timber  at  the  outlet 
of  the  bottom  culvert,  in  order  to  keep  it  filled  with  water,  to 
prevent  the  injury  which  the  bottom  of  it  might  receive  from  the 
water  discharged  from  the  upper  culverts. 

The  side  walls  which  retain  the  earth  at  the  entrance  to  the 
culverts,  should  be  arranged  with  grooves  to  receive  pieces  of 
scantling  laid  horizontally  between  the  walls,  termed  stop-planks, 
to  ^orm  a  temporary  dam,  and  cut  off  the  water  of  the  reser- 
voir, in  case  of  repairs  to  the  culverts,  or  to  the  face  of  the 
dam. 

The  valves  are  small  sliding  gates,  which  are  raised  and  low- 
ered by  a  rack  and  pinion,  or  by  a  square  screw.  The  cross 
section  of  the  culvert  is  contracted  by  a  partition,  either  of  ma- 
sonry or  timber,  at  the  point  where  the  valve  is  placed. 

701.  Dams  of  masonry  are  water-tight  walls,  of  suitable  forms 
and  dimensions  to  prevent  filtration,  and  resist  the  pressure  of 
water  in  the  reservoir.  The  most  suitable  cross  section  is  that 
of  a  trapezoid,  the  face  towards  the  water  being  vertical,  and 
the  exterior  face  inclined  with  a  suitable  batter  to  give  the  wall 
sufficient  stability.  The  wall  should  be  at  least  four  feet  thick 
at  the  water  line,  to  prevent  filtration,  and  this  thickness  may  be 
increased  as  circumstances  may  seem  to  require.  Buttresses 
should  be  added  to  the  exterior  facing,  to  give  the  wall  greater 
stability. 

702.  Suitable  dispositions  should  be  made  to  relieve  the  dam 
from  all  surplus  water  during  wet  seasons.  For  this  purpose 
arrangements  should  be  made  for  cutting  off  the  sources  of  sup- 
ply from  the  reservoir ;  and  a  cut,  termed  a  waste-weir,  (Fig. 
169,)  of  suitable  width  and  depth  should  be  made  at  some  point 
along  the  lop  of  the  dam,  and  be  faced  with  stone,  or  wood,  to 
give  an  outlet  to  the  water  over  the  dam.  In  high  dams  the 
total  fall  of  the  water  should  be  divided  into  several  partial  falls, 
by  dividing  the  exterior  surface  over  which  the  water  runs  into 
offsets.     To  break  the  shock  of  the  water  upon  the  horizontal 


326 


CANALS. 


surface  of  the  offset,  it  should  be  kept  covered  with  a  sheet 
water  retained  by  a  dam  placed  across  its  outlet. 


Fig.  169 — Represents  a  section  of  a  waste-weir  divided  into  two  falls. 

A,  body  of  the  dam. 

a,  top  of  the  waste-weir. 

6,  pool,  formed  by  a  stop-plank  dam  at  c,  to  break  the  fall  of  the  water. 

d,  covering  of  loose  stone  to  break  the  fall  of  the  water  from  the  pool  above. 

703.  In  extensive  reservoirs,  in  which  a  large  surface  is  ex- 
posed to  the  action  of  the  winds,  waves  might  be  forced  over 
the  top  of  the  dam,  and  subject  it  to  danger ;  in  such  cases  the 
precaution  should  be  taken  of  placing  a  parapet  wall  towards 
the  outer  edge  of  the  top  of  the  dam,  and  facing  the  top  through- 
out with  flat  stones  laid  in  mortar. 

704.  Lift  of  locks.  From  the  preceding  observations  on  the 
expenditure  of  water  for  the  service  of  the  navigation,  it  appears 
that  isolated  locks  are  more  favorable  under  this  point  of  view 
than  locks  in  flights.  The  engineer  is  not,  however,  always  left 
free  to  select  between  the  two  systems ;  for  the  form  of  the 
natural  surface  of  the  ground  may  compel  him  to  adopt  a  flight 
of  locks  at  certain  points.  As  to  the  comparative  expense  of  the 
two  methods,  a  flight  is  »in  most  cases  cheaper  than  the  same 
number  of  single  locks,  as  there  are  certain  parts  of  the  masonry 
which  can  be  suppressed.  There  is  also  an  economy  in  the 
suppression  of  the  small  gates,  which  are  not  needed  in  flights. 
It  is,  however,  more  difficult  to  secure  the  foundations  of  com- 
bined than  of  single  locks  from  the  effects  of  the  water,  which 
forces  its  way  from  the  upper  to  the  lower  level  under  the  locks. 
Where  an  active  trade  is  carried  on,  a  double  flight  is  sometimes 
arranged  ;  one  for  the  ascending,  the  other  for  the  descending 
boats.  In  this  case  the  water  which  fills  one  flight  may,  after 
».he  passage  of  the  boat,  be  partly  used  for  the  other,  by  an 
arrangement  of  valves  made  in  the  side  wall  separating  the 
locks. 

The  lift  of  locks  is  a  subject  of  importance,  both  as  regards 
the  consumption  of  water  for  the  navigation,  and  the  economy 
of  construction.  Locks  with  great  lifts,  as  may  be  seen  from 
the  remarks  on  the  passage  of  boats,  consume  more  water  than 
those  with  small  lifts.     They  require  also  more  care  in  thcii 


CANALS  327 

construction,  to  preserve  them  from  accidents,  owing  to  the  grea* 
pressure  of  water  against  their  sides.  The  expense  of  construc- 
tion is  otherwise  in  their  favor;  that  is,  the  expense  will  increase 
with  the  total  number  of  locks,  the  height  to  be  ascended  being 
the  sam<?.  The  smallest  lifts  are  seldom  less  than  five  feet,  and 
the  greatest,  for  ordinary  canals,  not  over  twelve  ;  medium  lifts 
of  seven  or  eight  feet  are  considered  the  best  under  every  point 
cf  view.  This  is  a  point,  however,  which  cannot  be  settled 
arbitrarily,  as  the  nature  of  the  foundations,  the  materials  used, 
the  embankments  around  the  locks,  the  changes  in  the  direction 
of  the  canal,  caused  by  varying  the  lifts,  are  so  many  modifying 
causes,  which  should  be  carefully  weighed  before  adopting  a 
definitive  plan. 

The  lifts  of  a  flight  should  be  the  same  throughout;  but  in 
isolated  locks  the  lifts  may  vary  according  to  circumstances.  If 
the  supply  of  water  from  the  summit  level  requires  to  be  econo- 
mized with  care,  the  lifts  of  locks  which  are  furnished  from  it 
may  be  less  than  those  lower  down. 

705.  Levels.  The  position  and  the  dimensions  of  the  levels 
must  be  mainly  determined  by  the  form  of  the  natural  surface. 
Those  points  are  naturally  chosen  to  pass  from  one  level  to 
another,  or  as  the  positions  for  the  locks,  where  there  is  an  ab- 
rupt change  in  the  surface. 

A  level,  by  a  suitable  modification  of  its  cross  section,  can  be 
made  as  short  as  may  be  deemed  desirable  ;  there  being  but  one 
point  to  be  attended  to  in  this,  which  is,  that  a  boat  passing  be- 
tween the  two  locks,  at  the  ends  of  the  level,  will  have  time  to 
enter  either  lock  before  it  can  ground,  on  the  supposition,  that 
the  water  drawu  off  to  fill  the  lower  lock,  while  the  boat  is  tra- 
versing the  level,  will  just  reduce  the  depth  to  the  draught  of  the 
boat. 

706.  Locks.  A  lock  (Fig.  170)  may  be  divided  into  three 
distinct  parts  : — 1st.  The  pait  included  between  the  two  gates, 
which  is  termed  the  chamber.  2d.  The  part  above  the  uppei 
gates,  termed  the  fore,  or  head-bay.  3d.  The  part  below  the 
lower  gates,  termed  the  aft,  or  tail-bay. 

707.  The  lock  chamber  must  be  wide  enough  to  allow  an 
easy  ingress  and  egress  to  the  boats  commonly  used  on  the  ca- 
nal; a  surplus  width  of  one  foot  over  the  width  of  the  boat  across 
the  beam  is  usually  deemed  sufficient  for  this  purpose.  The 
length  of  the  chamber  should  be  also  regulated  by  that  of  the 
boats  ;  it  should  be  such,  that  when  the  boat  enters  the  lock 
from  the  lower  level,  the  tail-gates  may  be  shut  without  requiring 
the  boat  to  unship  its  rudder. 

The  plan  of  the  chamber  is  usually  rectangular,  as  this  form 
u»,  in  every  respect,  superior  to  all  others.     In  the  cross  section 


326 


CANALS. 


I     1 

I 


s        -\ 


i 


H 


Fig.  170— Represents  a  plan  M,  and  a  section  N,  through  the  axis  of  a  single  lock  laid  on  a  be- 
ton  foundation. — A,  lock-chamber.  B,  fore-bay.  C,  tail-bay.  o,  a.  chamber-walls,  ft,  ft  re- 
cesses or  chambers  in  the  side  walls  for  upper-gates.  e,e,  lower-gate  chambers,  d,  d,  lift 
wall  and  upper  mitre  sill,  e,  e,  lower  mitre  sill.  A,  ft,  tail  walls,  o,  o,  head  walls.  «,  a, 
upper  wing,  or  return  walls,    n,  n,  lower  wing  walls.    D,  body  of  masonry  under  the  fore-ba>v 


CANALS. 


32t 


of  the  chamber,  (Fig.  171,)  the  sides  receive  generally  a  slight 


Fig.  171— Represents  a  section  of  Fig.  170,  through 


1 


1A||         the  chamber. 

I 


I?     1|     B,  chamber  formed  with  an  inverted-arch  bottom 

Ul 


H     l|      A,  A,  chamber  walls. 

P     1      " 


batter ;  as  when  so  arranged  they  are  found  to  give  greater  fa- 
cility to  the  passage  of  the  boat  than  when  vertical.  The  bot- 
tom of  the  chamber  is  either  flat  or  curved  ;  more  water  will  be 
required  to  fill  the  flat-bottom,ed  chamber  than  the  curved,  but  it 
will  require  less  masonry  in  its  construction. 

708.  The  chamber  is  terminated  just  within  the  head  gates  by 
a  vertical  wall,  the  plan  of  which  is  usually  curved.  As  this 
wall  separates  the  upper  from  the  lower  level,  it  is  termed  the 
lift-wall ;  it  is  usually  of  the  same  height  as  the  lift  of  the  lev- 
els. The  top  of  the  lift-wall  is  formed  of  cut  stone,  the  vertical 
joints  of  which  are  normal  to  the  curved  face  of  the  wall ;  this 
top  course  projects  from  six  to  nine  inches  above  the  bottom  of 
the  upper  level,  presenting  an  angular  point,  for  the  bottom  of 
the  head-gates,  when  shut,  to  rest  against.  This  is  termed  the 
mitre-sill.  Various  degrees  of  opening  have  been  given  to  the 
angle  between  the  two  branches  of  the  mitre-sill ;  it  is,  however, 
generally  so  determined,  that  the  perpendicular  of  the  isosceles 
triangle,  formed  by  the  two  branches,  shall  vary  between  on& 
fifth  and  one  sixth  of  the  base. 

As  stone  mitre-sills  are  liable  to  injury  from  the  shock  of  the 
gate,  they  are  now  usually  constructed  of  timber,  (Fig.  172,)  by 


Fig.  172— Represents  a  plan  of  a  wooden  mitre- 
sill,  and  a  horizontal  section  of  a  lock-gate 
(Fig.  17."<)  closed. 

a,  a,  mitre-sill  framed  with  the  pieces  b  and  c, 
and  firmly  fastened  to  the  side  walls  A,  A. 

d,  section  of  quoin  posts  of  lock-gate. 

e,  section  of  mitre  posts. 


framing  two  strong  beams  with  the  proper  angle  for  the  gate 
when  closed,  and  securing  them  firmly  upon  the  top  of  the  lift- 
wail.  It  will  be  well  to  place  the  top  of  the  mitre-sill  on  the 
lift-wall  a  little  lower  than  the  bottom  of  the  canal,  to  preserve 
it  from  being  struck  by  the  keel  of  the  boat  on  entering,  or 
leaving  the  lock. 

709.  The  cross  section  of  the  chamber  walls  is  usually  trape- 
zoidal ;  the  facing  receives  a  slight  batter.     The  chamber  walls 

42 


830  CANALS 

ire  exposed  to  two  opposite  efforts  ;  the  watei  in  the  loot  jt 
one  side,  and  the  embankment  against  the  wall  on  the  other. 
The  pressure  of  the  embankment  is  the  greater  as  well  as  the 
more  permanent  effort  of  the  two.  The  dimensions  of  the  wall 
must  be  regulated  by  this  pressure.  The  usual  manner  of  doing 
this,  is  to  make  the  wall  four  feet  thick  at  the  water  line  of  the 
upper  level,  to  secure  it  against  filtration ;  and  then  to  determine 
the  base  of  the  batter,  so  that  the  mass  of  masonry  snail  present 
sufficient  stability  to  counteract  the  tendency  of  the  pressure. 
The  spread,  and  other  dimensions  of  the  foundations,  will  be 
regulated  according  to  the  nature  of  the  soil,  in  the  same  way 
as  in  other  structures. 

710.  The  bottom  of  the  chamber,  as  has  been  stated,  may  be 
either  flat  or  curved.  The  flat  bottom  is  suitable  to  very  firm 
soils,  which  will  neither  yield  to  the  vertical  pressure  of  the 
chamber  walls,  nor  admit  the  water  to  filter  from  the  upper  level 
under  the  bottom  of  the  lock.  In  either  of  the  contrary  cases, 
the  bottom  should  be  made  with  an  inverted  arch,  as  this  form 
will  oppose  greater  resistance  to  the  upward  pressure  of  the 
water  under  the  bottom,  and  will  serve  to  distribute  the  weight 
of  the  walls  over  the  portion  of  the  foundation  under  the  arch. 
The  thickness  of  the  masonry  of  the  bottom  will  depend  on  the 
width  of  the  chamber,  and  the  nature  of  the  soil.  Were  the 
soil  a  solid  rock,  no  bottoming  would  be  requisite  ;  if  it  is  of  soft 
mud,  a  very  solid  bottoming,  from  three  to  six  feet  in  thickness, 
might  be  requisite. 

711.  The  principal  danger  to  the  foundations  arises  from  the 
water  which  may  filter  from  the  upper  to  the  lower  level,  under 
the  bottom  of  the  lock.  One  preventive  for  this,  but  not  an  ef- 
fectual one,  is  to  drive  sheeting  piles  across  the  canal  at  the  md 
of  the  head-bay ;  another,  which  is  more  expensive,  but  more 
certain  in  its  effects,  consists  in  forming  a  deep  trench  of  two  or 
three  feet  in  width,  just  under  the  head-bay,  and  filling  it  with 
beton,  which  unites  at  top  with  the  masonry  of  the  head-bay. 
Similar  trenches  might  be  placed  under  the  chamber  were  it 
considered  necessary. 

712.  The  lift-wall  usually  receives  the  same  thickness  as  the 
chamber  walls ;  but,  unless  the  soil  is  very  firm,  it  would  be 
more  prudent  to  form  a  general  mass  of  masonry  under  the  en- 
tire head-bay,  to  a  level  with  the  base  of  the  chamber  founda- 
tions, of  which  mass  the  lift-wall  should  form  a  part. 

713.  The  head-bay  is  enclosed  between  two  parallel  walls, 
which  form  a  part  of  the  side  walls  of  the  lock.  They  are  ter- 
minated by  two  wing  walls,  which  it  will  be  found  most  eco- 
nomical to  run  back  at  right  angles  with  the  side  walls.  A  re- 
cess,  termed  the  gate-chamber,  is  made-  ji  the  wall  of  the  head- 


CANALS.  331 

bav :  the  depth  of  this  recess  should  be  sufficient  to  allow  the 
gate,  when  open,  to  fall  two  or  three  inches  within  the  facing  of 
the  wall,  so  that  it  may  be  out  of  the  way  when  a  boat  is  pass- 
ing ;  the  length  of  the  recess  should  be  a  few  inches  more  than 
the  width  of  the  gate.  That  part  of  the  recess  where  the  gate 
turns  on  its  pivot  is  termed  the  hollow  quoin ;  it  receives  what 
is  termed  the  heel,  or  quoin-post  of  the  gate,  which  is  made  of  a 
suitable  form  to  fit  the  hollow  quoin.  The  distance  between  the 
hollow  quoins  and  the  face  of  the  lift-wall  will  depend  on  the 
pressure  against  the  mitre-sill,  and  the  strength  of  the  stone ; 
eighteen  inches  will  generally  be  found  amply  sufficient. 

The  side  walls  need  not  extend  more  than  twelve  inches  be 
yond  the  other  end  of  the  gate-chamber.  The  wing  walls  may 
be  extended  back  to  the  total  width  of  the  canal,  but  it  will  be 
more  economical  to  narrow  the  canal  near  the  lock,  and  to  ex- 
tend the  wing  walls  only  about  two  feet  into  the  banks,  or  sides. 
The  dimensions  of  the  side  and  wing  walls  of  the  head-bay  are 
regulated  in  the  same  way  as  the  chamber  walls. 

The  bottom  of  the  head-bay  is  flat,  and  on  the  same  level  with 
the  bottom  of  the  canal ;  the  exterior  course  of  stones  at  the  en- 
trance to  the  lock  should  be  so  jointed  as  not  to  work  loose. 

714.  The  gate-chambers  for  the  lower  gates  are  made  in  the 
chamber  walls  ;  and  it  is  to  be  observed,  that  the  bottom  of  the 
chamber,  where  the  gates  swing  back,  should  be  flat,  or  be  oth- 
erwise arranged  not  to  impede  the  play  of  the  gates. 

715.  The  side  walls  of  the  tail-bay  are  also  a  part  of  the  gen- 
eral side  walls,  and  their  thickness  is  regulated  as  in  the  prece- 
ding cases.  Their  length  will  depend  chiefly  on  the  pressure 
which  the  lower  gates  throw  against  them  when  the  lock  is  full; 
and  partly  on  the  space  required  by  the  lock-men  in  opening  and 
shutting  gates  manoeuvred  by  the  balance  beam.  A  calculation 
must  be  made  for  each  particular  case,  to  ascertain  the  most 
suitable  length.  The  side  walls  are  also  terminated  by  wing 
walls,  similarly  arranged  to  those  of  the  head-bay.  The  points 
of  junction  between  the  wing  and  side  walls  should,  in  both 
cases,  either  be  curved,  or  the  stones  at  the  angles  be  rounded 
off.  One  or  two  perpendicular  grooves  are  sometimes  made  in 
the  side  walls  of  the  tail-bay,  to  receive  stop-planks,  when  a 
temporary  dam  is  needed,  to  shut  oil' the  water  of  the  lower  level 
from  the  chamber,  in  case  of  repairs,  &c.  Similar  arrangements 
might  be  made  at  the  head-bay,  but  they  are  not  indispensable 
in  either  case. 

The  strain  on  the  walls  at  the  holl  w  quoins  is  greater  than  ai 
any  other  points,  owing  to  the  pressure  at  those  points  from  the 
gales,  when  they  are  shut,  and  to  the  action  of  the  gates  whec 
in  motion ;    to  counteract  this,  and  strengthen  the  walls,  bu4.- 


332  CANALS. 

tresses  should  be  placed  at  the  back  of  the  walls,  in  the  most 
favorable  position  behind  the  quoins  to  subserve  the  object  in 
view. 

The  bottom  of  the  tail-bay  is  arranged,  in  all  respects,  like 
that  of  the  head-bay. 

716.  The  top  of  the  side  walls  of  the  lock  may  be  from  one 
to  two  feet  above  the  general  level  of  the  water  in  the  upper 
reach ;  the  top  course  of  the  masonry  being  of  heavy  large 
blocks  of  cut  stone,  although  this  kind  of  coping  is  not  indis- 
pensable, as  smaller  masses  have  been  found  to  suit  the  same 
purpose,  but  they  are  less  durable.  As  to  the  masonry  of  the 
lock,  in  general,  it  is  only  necessary  to  observe,  that  those  parts 
alone  need  be  of  cut  stone  where  there  is  great  wear  and  tear 
from  any  cause,  as  at  the  angles  generally ;  or  where  an  accu- 
rate finish  is  indispensable,  as  at  the  hollow  quoins.  The  other 
parts  may  be  of  brick,  rubble,  beton,  &c,  but  every  part  should 
be  laid  in  the  best  hydraulic  mortar. 

717.  The  filling  and  emptying  the  lock  chamber  have  given 
rise  to  various  discussions  and  experiments,  all  of  which  have 
been  reduced  to  the  comparative  advantages  of  letting  the  watei 
in  and  off  by  valves  made  in  the  gates  themselves,  or  by  culverts 
in  the  side  walls,  which  are  opened  and  shut  by  valves.  When 
the  water  is  let  in  through  valves  in  the  gates,  its  effects  on  the 
sides  and  bottom  of  the  chamber  are  found  to  be  very  injurious, 
particularly  in  high  lift-walls ;  besides  the  inconvenience  result- 
ing from  the  agitation  of  the  boat  in  the  lock.  To  obviate  this, 
in  some  degree,  it  has  been  proposed  to  give  the  lift-wall  the 
form  of  an  inclined  curved  surface,  along  which  the  water  might 
descend  without  producing  a  shock  on  the  bottom. 

718.  The  side  culverts  are  small  arched  conduits,  of  a  circu- 
lar, or  an  elliptical  cross  section,  which  are  made  in  the  mass 
of  masonry  of  the  side-walls,  to  convey  the  water  from  the  up- 
per level  to  the  chamber.  These  culverts,  in  some  cases,  run 
the  entire  length  of  the  side  walls,  on  a  level  with  the  bottom 
of  the  chamber,  from  the  lift-wall  to  the  end  of  the  tail-wall,  and 
have  several  outlets  leading  to  the  chamber.  They  are  arranged 
with  two  valves,  one  to  close  the  mouth  of  the  culvert,  at  the 
upper  level,  the  other  to  close  the  outlet  from  the  chamber,  to 
the  lower  level.  This  is,  perhaps,  one  of  the  best  arrangements 
for  side  culverts.  They  all  present  the  same  difficulty  in  making 
repairs  when  oui  of  oi  der,  and  they  are  moreover  very  subject 
to  accidents.  They  are  therefore  on  these  accounts  inferior  to 
valves  in  the  gates. 

719.  It  has  also  been  proposed,  to  avoid  the  inconveniences 
of  culverts,  and  the  disadvantages  of  lift- walls,  by  suppressing 
the  latter,  and  gradually  increasing  the  depth  of  the  upper  level« 


CANALS 


333 


to  the  bottom  of  the  chamber.  Tin  3  method  presents  a  saving 
in  the  mass  of  masonry,  but  the  gates  will  cost  more,  as  the 
head  and  tail  gates  must  be  of  the  same  height.  It  would  en- 
tirely remove  the  objection  to  valves  in  the  gates,  as  the  current 
through  them,  in  this  case,  would  not  be  sufficiently  strong  to 
injure  the  masonry. 

720.  The  bottom  of  the  canal  below  the  lock  should  be  pro  - 
tected  by  what  is  termed  an  apron,  which  is  a  covering  of  plank 
laid  on  a  grillage,  or  else  one  of  brush-wood  and  dry  stone.  The 
sides  should  also  be  faced  with  timber  or  dry  stone.  The  length 
of  this  facing  will  depend  on  the  strength  of  the  current ;  gene- 
rally not  more  than  from  fifteen  to  thirty  feet  from  the  lock  will 
require  it.  The  entrance  to  the  head-bay  is,  in  some  cases, 
similarly  protected,  but.  this  is  unnecessary,  as  the  current  has 
but  a  very  slight  effect  at  that  point. 

721.  Locks  constructed  of  timber  and  dry  stcne,  termed  com- 
posite-locks, are  to  be  met  with  on  several  of  the  canals  of  the 
United  States.  The  side  walls  are  formed  of  dry  stone  carefully 
laid ;  the  sides  of  the  chamber  being  faced  with  plank  nailed  to 
horizontal  and  upright  timbers,  which  are  firmly  secured  to  the 
dry  stone  walls.  The  walls  rest  upon  a  platform  laid  upon  heavy 
beams  placed  transversely  to  the  axis  of  the  lock.  The  bottom 
of  the  chamber  usually  receives  a  double  thickness  of  plank 
The  quoin-posts  and  mitre-sills  are  formed  of  heavy  beams. 

722.  Lock  Gates.     A  lock  gate  (Fig.  173)  is  composed  of  two 


Fig.  173— Repre- 
sents the  eleva- 
tion of  a  lock- 
gate  closed. 

a,  a,  quoin-posts. 

b,  mitre-posts. 

c,  c,  cross  pieces 
framed  into  a 
and  b  and  firmly 
connected  with 
them  by  wrought 
iron  plates. 

o,  plank  or  sheath- 
ing of  the  gate. 

d,  valve. 

m.  m,  balance- 
beam. 


leaves,  each  leaf  consisting  of  a  solid  frame-work  covered  on 
the  side  towards  the  water  with  thick  plank  made  water-tight. 
The  frame  usually  consists  of  two  uprights,  of  several  horizon- 
tal cross  pieces  let  into  the  uprights,  and  sometimes  a  diagonal 
piece,  or  brace,  intended  to  keep  the  frame  of  an  invariable 


334  CANALS. 

form,  is  added.  The  upright,  around  which  the  leaf  turns,  termed 
the  quoin  or  heel-post,  is  rounded  off  on  the  back  to  fit  in  the 
hollow  quoin ;  it  is  made  slightly  eccentric  with  it,  so  that  it  may 
turn  easily  without  rubbing  against  the  quoin  ;  its  lower  end  rests 
on  an  iron  gudgeon,  to  which  it  is  fitted  by  a  corresponding  in 
dentation  in  an  iron  socket  on  the  end  ;  the  upper  extremity  is 
secured  to  the  side  walls  by  an  iron  collar,  within  which  the  post 
turns.  The  collar  is  so  arranged  that  it  can  be  easily  fastened 
to,  or  loosened  from  two  iron  bars,  termed  anchor-irons,  which 
are  firmly  attached  by  bolts,  or  a  lead  sealing,  to  the  top  course 
of  the  walls.  One  of  the  anchor-irons  is  placed  in  a  line  with 
the  leaf  when  shut,  the  other  in  a  line  with  it  when  open,  to  re  • 
sist  most  effectually  the  strain  in  those  two  positions  of  the  gate. 
The  opposite  upright,  termed  the  mitre-post,  has  one  edge  bev- 
elled off,  to  fit  against  the  mitre-post  of  the  other  leaf  of  the 
gate. 

723.  A  long  heavy  beam,  termed  a  balance  beam,  from  its 
partially  balancing  the  weight  of  the  leaf,  rests  on  the  quoin 
post,  to  which  it  is  secured,  and  is  mortised  with  the  mitre  post. 
The  balance  beam  should  be  about  four  feet  above  the  top  of  the 
lock,  to  be  readily  manoeuvred  ;  its  principal  use  being  to  open 
and  shut  the  leaf. 

724.  The  top  cross  piece  of  the  gate  should  be  about  on  a 
level  with  the  top  of  the  lock ;  the  bottom  cross  piece  should 
swing  clear  of  the  bottom  of  the  lock.  The  position  of  the  in- 
termediate cross  pieces  may  be  made  to  depend  on  their  dimen- 
sions :  if  they  are  of  the  same  dimensions,  they  should  be  placed 
nearer  together  at  the  bottom,  as  the  pressure  of  the  water  is 
there  greatest ;  but,  by  making  them  of  unequal  dimensions, 
they  may  be  placed  at  equal  distances  apart  ;  this,  however,  is 
not  of  much  importance  except  for  large  gates,  and  considerable 
depths  of  water. 

The  plank  may  be  arranged  either  parallel  to  the  uprights,  or 
parallel  to  the  diagonal  brace  ;  in  the  latter  position  they  will  act 
with  the  brace  to  preserve  the  form  of  the  frame. 

725.  A  wide  board  supported  on  brackets,  is  often  affixed  to 
the  gates,  both  for  the  manoeuvre  of  the  machinery  of  the  valves, 
and  to  serve  as  a  foot  bridge  across  the  lock.  The  valves  are 
small  gates  which  are  arranged  to  close  the  openings  made  in 
the  gates  for  letting  in,  or  drawing  off  the  water.  They  are  ar- 
ranged to  slide  up  and  down  in  grooves,  by  the  aid  of  a  rack  and 
pinion,  or  a  square  screw  ;  or  they  may  be  made  to  open  or  shut 
by  turning  on  a  vertical  axis,  in  which  case  they  are  termed  pad- 
dle gates.  The  openings  in  the  upper  gates  are  made  between 
the  two  lowest  cross  pieces.  In  the  lower  gates  the  openings 
are  placed  just  below  the  surface  of  the  water  in  the  reach.  The 


CANALS.  335 

»ize  of  the  opening  will  depend  on  the  time  in  which  it  is  r« 
quired  to  fiU  the  lock. 

726.  Accessory  Works.  Under  this  head  are  classed  those 
constructions  which  are  not  a  part  of  the  canal  proper,  although 
generally  found  necessary  on  all  canals  :  as  the  culverts  for  con- 
veying off  the  water  courses  which  intersect  the  line  of  the  canal; 
the  inlets  of  feeders  for  the  supply  of  water  ;  aqueuuel  bridges, 
&c.  &c. 

727.  Culverts.  The  disposition  to  be  made  of  water  courses 
intersecting  the  line  of  the  canal  will  depend  on  their  size,  the 
character  of  their  current,  and  the  relative  positions  of  the  canal 
and  stream. 

Small  biooks  which  lie  lower  than  the  canal  may  be  conveyed 
under  it  through  an  ordinary  culvert.  If  the  level  of  the  canal 
and  brook  is  naarly  the  same,  it  will  then  be  necessary  to  make 
the  culvert  in  the  shape  of  an  inverted  syphon,  and  it  is  therefore 
termed  a  broken-back  culvert.  If  the  water  of  the  brook  is 
generally  limpid,  and  its  current  gentle,  it  may,  in  the  last  case, 
be  received  into  the  canal.  The  communication  of  the  brook,  or 
feeder,  with  the  canal,  should  be  so  arranged  that  the  water  may 
be  shut  off,  or  let  in  at  pleasure,  in  any  quantity  desired.  For 
this  purpose  a  cut  is  made  through  the  side  of  the  canal,  and  the 
sides  and  bottom  of  the  cut  are  faced  with  masonry  laid  in  hy- 
draulic mortar.  A  sliding  gate,  fitted  into  two  grooves  made  in 
the  side  walls,  is  manoeuvred  by  a  rack  ?nd  pinion,  so  as  to  reg- 
ulate the  quantity  of  water  to  be  let  in.  The  water  of  the  feeder, 
or  brook,  should  first  be  received  in  a  basin,  or  reservoir,  near 
the  canal,  where  it  may  deposite  its  sediment  before  it  is  drawn 
off.  In  cases  where  the  line  of  the  canal  is  crossed  by  a  torrent, 
which  brings  down  a  large  quantity  of  sand,  pebbles,  &c,  it  may 
be  necessary  to  make  a  permanent  structure  over  the  canal,  form- 
ing a  channel  for  the  torrent ;  but  if  the  discharge  of  the  torrent 
is  only  periodical,  a  moveable  chonn^l  may  be  arranged,  for  the 
same  purpose,  by  constructing  a  boat  with  a  deck  and  sides  to 
form  the  water-way  of  the  torreni.  The  boat  is  kept  in  a  recess 
in  the  canal  near  the  point  where  it  is  used,  and  is  floated  to  its 
position,  and  sunk  when  wanted. 

728.  Aqueducts,  <$-c.    When  the  line  of  the  canai  is  intersect- 
ed by  a  wide  Water-course,  the  communication  between  the  two 
shores  must  be  effected  either  by  a  canal  pqueduct  bridge,  or  by 
the  boats  descending  from  the  canal  into  the  stream.     As  the 
construction  of  aqueduct  bridges  has  already  been  considered 
nothing  farther  on  this  point  need  here  be   added.     The  expe 
dient  of  crossing  the  stream  by  the  boats  may  be  tended  wit* 
many  grave  inconveniences  in  water  courses  Hahl*  fo  frea^vt0 
or  to  considerable  variations  of  ?evel  at  differen*   9«*v^on«.     I* 


3o6  CANALS. 

these  cases  locks  must  be  so  arranged  on  each  side,  where  the 
canal  enters  the  stream,  that  boats  may  pass  from  the  one  to  the 
other  under  all  circumstances  of  difference  of  level  between  the 
two.  The  locks  and  the  portions  of  the  canal  which  join  the 
stream  must  be  secured  against  damage  from  freshets  by  suita- 
ble embankments  ;  and,  when  the  summer  water  of  the  stream 
is  so  low  that  the  navigation  would  be  impeded,  a  dam  across 
the  stream  will  be  requisite  to  secure  an  adequate  depth  of  water 
during  this  epoch. 

729.  Canal  Bridges.  Bridges  for  roads  over  a  canal,  termed 
canal-bridges,  are  constructed  like  other  structures  of  the  same 
kind.  In  planning  them  the  engineer  should  endeavor  to  give 
sufficient  height  to  the  bridge  to  prevent  those  accidents,  of  but 
too  frequent  occurrence,  from  persons  standing  upright  on  the 
deck  of  the  passage-boat  while  passing  under  a  bridge. 

730.  Waste-Wier.  Waste-wiers  must  be  made  along  the 
levels  to  let  off  the  surplus  water.  The  best  position  for  them 
is  at  points  where  they  can  discharge  into  natural  water  courses. 
The  best  arrangement  for  a  waste-wier  is  to  make  a  cut  through 
the  side  of  the  canal  to  a  level  with  the  bottom  of  it,  so  that,  in 
case  of  necessity,  the  waste-wier  may  also  serve  for  draining  the 
level.  The  sides  and  bottom  of  the  cut  must  be  faced  with  ma- 
sonry, and  have  grooves  left  in  them  to  receive  stop-plank,  or  a 
sliding  gate,  over  which  the  surplus  water  is  allowed  to  flow, 
under  the  usual  circumstances,  but  which  can  be  removed,  if  it 
be  found  necessary,  either  to  let  off  a  larger  amount  of  water,  or 
to  drain  the  level  completely. 

731.  Temporary  Dams.  In  long  levels  an  accident  happen- 
ing at  any  one  point  might  cause  serious  injury  to  the  navigation, 
besides  a  great  loss  of  water.  To  prevent  this,  in  some  meas- 
ure, the  width  of  the  canal  may  be  diminished,  at  several  points 
of  a  long  level,  to  the  width  of  a  lock,  and  the  sides,  at  these 
points,  may  be  faced  with  masonry,  arranged  with  grooves  and 
stop-planks,  to  form  a  temporary  dam  for  shutting  off  the  water 
on  either  side. 

732.  Tide,  or  Guard  Lock.  The  point  at  which  a  canal  en- 
ters a  river  requires  to  be  selected  with  judgment.  Generally 
speaking,  a  bar  will  be  found  in  the  principal  water  course  at, 
or  below,  the  points  where  it  receives  its  affluents.  W  hen  the 
canal,  therefore,  follows  the  valley  of  an  affluent,  its  outlet 
should  be  placed  below  the  bar,  tu  render  its  navigation  perma- 
r.ently  secure  from  obstruction.  A  large  basin  is  usually  formed 
at  the  outlet,  for  the  convenience  of  commerce  ;  and  the  entrance 
from  this  basin  to  the  canal,  or  from  the  river  to  the  basin,  is  ef- 
fected by  means  of  a  lock  with  double  gates,  so  arranged  that  a 
boat  can  be  passed  either  way,  according  as  the  level  in  the  one 


CANALS.  337 

is  higher  or  lower  than  that  in  the  other.  A  lock  so  arranged  is 
termed  a  tide,  or  guard  lock,  from  its  uses.  The  position  of 
the  tail  of  this  lock  is  not  indifferent  in  all  cases  where  it  forms 
the  outlet  to  the  river ;  for  were  the  tail  placed  up  stream,  it 
would  be  more  difficult  to  pass  in  or  out,  than  if  it  were  down 
stream. 

733.  The  general  dimensions  of  canals  and  their  locks  in  this 
country  and  in  Europe,  with  occasional  exceptions,  do  not  differ 
in  any  considerable  degree. 

English  Canals.  Two  classes  of  canals  are  to  be  met  with  in 
England,  differing  materially  in  their  dimensions.  The  following 
are  the  usual  dimensions  of  the  cross  section  of  the  largest  size, 
and  those  of  their  locks  :  — 

Width  of  section  at  the  water  level,  from  36  to  40  feet. 
Width  at  bottom,  ....         24     " 

Depth,  5     " 

Length  of  lock  between  mitre-sills,  75  to  85     " 

Width  of  chamber,  .         .         .         .          15     '• 

The  Caledonian  canal,  in  Scotland,  which  connects  Loch-Eil 
on  the  Western  sea  with  Murray  Firth  on  the  Eastern,  is  re- 
markable for  its  size,  which  will  admit  of  the  passage  of  frigates 
of  the  second  class.  The  following  are  the  principal  dimensions 
of  the  cross  section  of  the  canal  and  its  locks  : — 


Width  of  canal  at  the  water  level, 

110  feet. 

Width  at  bottom, 

50     " 

Depth  of  water,             .         .         .         . 

20     " 

Width  of  berm,        .... 

6     " 

Length  of  lock  between  mitre-sills, 

180     " 

Width  of  chamber  at  top, 

40      ' 

Lift  of  lock,         .         .         .         .         . 

8     " 

The  side  walls  of  the  locks  are  built  with  a  curved  batter , 
they  are  of  the  uniform  thickness  of  6  feet,  and  are  strengthened 
by  counterforts,  placed  about  15  feet  apart,  which  are  4  feet  wide 
and  of  the  same  thickness.     The  bottom  of  the  chamber  is  form 
ed  with  an  inverted  arch. 

French  Canals.  In  France  the  following  uniform  system  has 
been  established  for  the  dimensions  of  canals  and  their  locks  •—  - 

Width  of  canal  at  water  level,  .         .         52  feet. 

Width  at  bottom,         .         .         .         .    33  to  36     " 
Depth  of  water,       .....  5     " 

Length  of  lock  between  mitre-sills,        .  115     " 

Width  of  lock,         .         .         .  .         17     " 

The  boats  adapted  to  these  dimensions  are  from  105  to  108 
feet  long,  16^  feet  across  the  beam,  and  have  a  draught  of  4  feet. 

43 


338  CANALS. 

The  English  and  French  canals  usually  have  but  one  tow-path 
which  is  from  9  to  1 2  feet  wide,  and  about  2  feet  above  the  wa 
ter  level.  The  side  of  the  tow-path  embankment  next  to  th« 
water-way  is  usually  faced  either  with  dry  stone,  masonry,  o) 
planks  retained  by  short  piles. 

Canals  of  the   United  States  and  Canada.    The  original  di 
mensions  of  the  New- York  Erie  canal  and  its  locks,  have  been 
generally  adopted  for  similar  works  subsequently  constructed  in 
most  of  the  other  states.     The  dimensions  of  this  canal  and  its 
locks  are  as  follows  : — 

Width  of  canal  at  top, 

Width  at  bottom, 

Depth  of  water,        .... 

Width  of  tow-path, 

Length  of  locks  between  mitre-sills, 

Width  of  locks, 

For  the  enlargement  of  the  Erie  canal,  the  following  dimen- 
sions have  been  adopted  : — 

Width  of  canal  at  top, 

Width  at  bottom, 

Depth  of  water,       .... 

Width  of  tow-path, 

Length  of  locks  between  mitre-sills, 

Width  of  lock  at  top, 

Width  of  lock  at  bottom, 

Lift  of  locks,        .... 

Between  the  double  locks  a  culvert  is  placed,  which  allows 
the  water  to  flow  from  the  level  above  the  lock  to  the  one  below, 
when  there  is  a  surplus  of  water  in  the  former. 

A  well,  or  pit,  is  left  between  the  lift-wall  of  the  lock  and  the 
cross  wall  which  retains  the  earth  at  the  head  of  the  lock  to  the 
level  of  the  bottom  of  the  canal.  This  pit,  receiving  the  deposite 
of  sand  and  gravel  brought  down  by  the  current,  prevents  it  from 
obstructing  the  play  of  the  gates. 

On  the  Chesapeake  and  Ohio  canal,  the  cross  section  of  the 
canal  below  Harper's  Ferry  has  received  the  following  dimen- 
sions : 

Width  of  canal  at  top,      ....         60  feet. 
Width  at  bottom,          ....  42     " 

Depth  of  water,       .....  6     " 

Length  of  locks  between  mitre-sills,      .  90     " 

Width  of  locks, 15     " 

The  following  dimensions  have  been  adopted  on  the  Jamet 
river  canal,  in  Virginia : — 


40  feet. 

28 

<( 

4 

it 

9  to  12 

ti 

90 

u 

15 

following 

<< 
di] 

70  feet. 

42 

«« 

7 

<« 

14 

u 

110 

(( 

18.8 

<« 

14.6 

M 

8 

<( 

CANAL8. 

Width  of  canal  at  top, 

50  ftet 

Width  at  bottom, 

30     " 

Depth  of  water,       .... 

Length  of  locks, 

Width  of  locks 

5     " 

100     " 

15     " 

j;-3 


The  Rideau  canal,  which  connects  Lake  Ontario  with  the 
River  Ottawa,  is  arranged  for  steam  navigation.  A  considerable 
portion  of  this  line  consists  of  slack-water  navigation,  formed  by 
connecting  the  natural  water-courses  between  the  outlets  of  the 
canal.  The  length  of  the  locks  on  this  canal  is  134  feet  between 
the  mitre-sills,  and  their  width  33  feet. 

The  Welland  canal,  between  lakes  Erie  and  Ontario,  as  origin- 
ally constructed,  received  the  following  dimensions  : — 

Width  of  canal  at  top,      ....         56  feet. 
Width  at  bottom,  ....  24     " 

Depth  of  water,       .....  8     " 

Length  of  locks  betwen  mitre-sills,         .  110     " 

Width  of  locks, 22     " 

The  canals  and  locks  made  to  avoid  the  dangerous  rapids  ol 
the  St.  Lawrence  are  in  all  respects  among  the  largest  in  the 
world.     The  following  are  the  dimensions  of  the  portion  of  the 
canal  and  the  locks  between  Long  Sault  and  Cornwall : — 
,         Width  of  canal  at  top,      ....       132  feet. 
Width  at  bottom,  .         .         .         .  100 

Depth  of  water,       .....  8 

Width  of  tow-path,       ....  12 

Length  of  locks  between  mitre-sills,  .       200 

Width  of  locks  at  top,  .         .         .  56.6 

Width  of  locks  at  bottom,  ...  43 
A  berm  of  5  feet  is  left  on  each  side  between  the  water  way 
and  the  foot  of  the  interior  slope  of  the  tow-path.  The  height 
of  the  tow-path  is  6  feet  above  the  berm.  By  increasing  the 
depth  of  water  in  the  canal  to  10  feet,  the  water  line  at  top  can 
be  increased  to  150  feet 


3-W*  RIVERS. 


RIVERS. 

734.  Natural  features  of  Rivers.  All  rivers  present  the  same 
natural  features  and  phenomena,  which  are  more  or  less  strongly 
marked  and  diversified  by  the  character  of  the  region  through 
which  they  flow.  Taking  their  rise  in  the  highlands,  and  gradu- 
ally descending  thence  to  some  lake,  or  sea,  their  beds  are  mod- 
ified by  the  nature  of  the  soil  of  the  valleys  in  which  they  lie, 
and  the  velocities  of  their  Currents  are  affected  by  the  same 
causes.  Near  their  sources,  their  beds  are  usually  rocky,  irregular, 
narrow,  and  steep,  and  their  currents  are  rapid.  Approaching 
their  outlets,  the  beds  become  wider  and  more  regular,  the  de- 
clivity less,  and  the  current  more  gentle  and  uniform.  In  the 
upper  portions  of  the  beds,  their  direction  is  more  direct,  and  the 
obstructions  met  with  are  usually  of  a  permanent  character,  aris- 
ing from  the  inequalities  of  the  bottom.  In  the  lower  portions, 
the  beds  assume  a  more  tortuous  course,  winding  through  their 
valleys,  and  forming  those  abrupt  bends,  termed  elbows,  which 
seem  subject  to  no  fixed  laws  ;  and  here  are  found  those  ob- 
structions, of  a  more  changeable  character,  termed  bars,  which, 
are  caused  by  deposites  in  the  bed,  arising  from  the  wear  of  the 
banks  by  the  current. 

735.  The  relations  which  are  found  to  exist  between  the  cross 
section  of  a  river,  its  longitudinal  slope,  the  nature  of  its  bed, 
and  its  volume  of  water,  are  termed  the  regimen  of  the  river. 
When  these  relations  remain  permanently  invariable,  or  change 
insensibly  with  time,  the  river  is  said  to  have  a  fixed  regimen. 

736.  Most  rivers  acquire  in  time  a  fixed  regimen,  although 
periodically,  and  sometimes  accidentally,  subject  to  changes 
from  freshets  caused  by  the  melting  of  snow,  and  heavy  falls  of 
rain.  These  variations  in  the  volume  of  water  thrown  into  the 
bed,  cause  corresponding  changes  in  the  velocity  of  the  current, 
and  in  the  form  and  dimensions  of  the  bed.  These  changes  will 
depend  on  the  character  of  the  soil,  and  the  width  of  the  valley. 
In  narrow  valleys,  where  the  banks  do  not  readily  yield  to  the 
action  of  the  current,  the  effects  of  any  variation  of  velocity  will 
only  be  temporarily  to  deepen  the  bed.  In  wide  valleys,  where 
the  soil  of  the  banks  is  more  easily  worn  by  the  current  than  the 
bottom,  any  increase  in  the  volume  of  water  will  widen  the  bed  ; 
and  if  one  bank  yields  more  than  the  other,  an  elbow  will  be 
formed,  and  the  position  of  the  bed  will  be  gradually  shifted  to- 
wards the  concave  side  of  the  elbow. 


RIVERS.  341 

737.  The  formation  of  elbows  occasions  also  variations  in  the 
depth  and  velocity  of  the  water.  The  greatest  depth  is  found 
at  the  concave  side.  At  the  straight  portions  which  connect  twc 
elbows,  the  depth  is  found  to  decrease,  and  the  velocity  of  the 
current  to  increase.  The  bottom  of  the  bed  thus  presents  a  se- 
ries of  undulations,  forming  shallows  and  deep  pools,  with  rapid 
tnd  gentle  currents. 

738.  Bars  are  formed  at  those  points,  where  from  any  cause 
the  velocity  of  the  current  receives  a  sudden  check.  The 
particles  suspended  in  the  water,  or  borne  along  over  the  bottom 
of  the  bed  by  the  current,  are  deposited  at  these  points,  and  con- 
tinue to  accumulate,  until,  by  the  gradual  filling  of  the  bed,  the 
water  acquires  sufficient  velocity  to  bear  farther  on  the  particles 
that  reach  the  bar,  when  the  river  at  this  point  acquires  and  re- 
tains a  fixed  regimen,  until  disturbed  by  some  new  cause. 

739.  The  points  at  which  these  changes  of  velocity  usually 
take  place,  and  near  which  bars  are  found,  are  at  the  junction  of 
a  river  with  its  affluents,  at  those  points  where  the  bed  of  the  river 
receives  a  considerable  increase  in  width,  at  the  straight  portions 
of  the  bed  between  elbows,  and  at  the  outlet  of  the  river  to  the 
sea.  The  character  of  the  bars  will  depend  upon  that  of  the  sol 
of  the  banks,  and  the  velocity  of  the  current.  Generally  speak- 
ing, the  bars  in  the  upper  portions  of  the  bed  will  be  composed 
of  particles  which  are  larger  than  those  by  which  they  are  formed 
lower  down.  These  accumulations  at  the  mouths  of  large  rivers 
form  in  time  extensive  shallows,  and  great  obstructions  to  the 
discharge  of  the  water  during  the  seasons  of  freshets.  The  river 
then,  not  finding  a  sufficient  outlet  by  the  ordinary  channel,  ex- 
cavates for  itself  others  through  the  most  yielding  parts  of  the 
deposites.  In  this  manner  are  formed  those  features  which  char- 
acterize the  outlets  of  many  large  rivers,  and  which  are  termed 
delta,  after  the  name  given  to  the  peculiar  shape  of  the  outlets 
of  the  Nile. 

740.  River  Improvements.  There  is  no  subject  that  falls  with- 
in the  province  of  the  engineer's  art,  that  presents  greater  diffi- 
culties and  more  uncertain  issues  than  the  improvement  of  rivers. 
Ever  subject  to  important  changes  in  their  regimen,  as  the  re- 
gions by  which  they  are  fed  are  cleared  of  their  forests  and 
brought  under  cultivation,  one  century  see3  them  deep,  flowing 
with  an  equable  current,  and  liable  only  to  a  gradual  increase  in 
volume  during  the  seasons  of  freshets ;  while  the  next  finds  their 
beds  a  prey  to  sudden  and  great  freshets,  which  leave  them,  after 
their  violent  passage,  obstructed  by  ever  shifting  bars  and  elbows. 
Besides  these  revolutions  brought  about  in  the  course  of  years, 
every  obstruction  temporarily  placed  in  the  way  of  the  current 
every  attempt  to  guard  one  point  from  its  action  by  any  artificial 


342  RIVERS 

means,  inevitably  produces  some  coi  responding  change  at  another 
which  can  seldom  be  foreseen,  and  for  which  the  remedy  applied 
may  prove  but  a  new  cause  of  harm.  Thus,  a  bar  removed  from 
one  point  is  found  gradually  to  form  lower  down ;  one  bank  pro- 
tected from  the  current's  force  transfers  its  action  to  the  opposite 
one,  on  any  increase  of  volume  from  freshets,  widening  the  bed, 
and  frequently  giving  a  new  direction  to  the  channel.  Owing  to 
these  ever  varying  causes  of  change,  the  best  weighed  plans  of 
river  improvement  sometimes  result  in  complete  failure. 

741.  In  forming  a  plan  for  a  river  improvement,  the  principal 
objects  to  be  considered  by  the  engineer,  are,  1st,  The  means  to 
be  taken  to  protect  the  banks  from  the  action  of  the  current. 
2d,  Those  to  prevent  inundations  of  the  surrounding  country. 
3d,  The  removal  of  bars,  elbows,  and  other  natural  obstructions 
to  navigation.  4th,  The  means  to  be  resorted  to  for  obtaining  a 
suitable  depth  of  water  for  boats,  of  a  proper  tonnage,  for  the 
trade  on  the  river. 

742.  Means  for  protecting  the  banks.  To  protect  the  banks, 
either  the  velocity  of  the  current  in-shore  must  be  decreased  so 
as  to  lessen  its  action  on  the  soil ;  or  else  a  facing  of  some  ma- 
terial sufficiently  durable  to  resist  its  action  must  be  employed. 
The  former  method  may  be  used  when  the  banks  are  low  and 
have  a  gentle  declivity.  The  simplest  plan  for  this  purpose  con- 
sists either  in  planting  such  shrubbery  on  the  declivity  as  will 
thrive  near  water  ;  or  by  driving  down  short  pickets  and  interla- 
cing them  with  twigs,  forming  a  kind  of  wicker-work.  These  coiv 
structions  break  t.lie  force  of  the  current,  and  diminish  its  velocity 
near  the  shore,  and  thus  cause  the  water  to  deposite  its  finer  par- 
ticles, which  gradually  fill  out  and  strengthen  the  banks.  If  the 
banks  are  high,  and  are  subject  to  cave  in  from  the  action  of  the 
current  on  their  base,  they  may  be  either  cut  down  to  a  gentle 
declivity,  as  in  the  last  case  ;  or  else  they  may  receive  a  slope 
of  nearly  45°,  and  be  faced  with  dry  stone,  care  being  taken  to 
secure  the  base  by  blocks  of  loose  stone,  or  by  a  facing  of  brush 
and  stone  laid  in  alternate  layers. 

743.  Measures  against  inundations.  At  the  points  in  the 
course  of  a  river  where  inundations  are  to  be  apprehended,  the 
water-way,  if  practicable,  should  be  increased ;  all  obstructions 
to  the  free  discharge  of  the  water  below  the  point  should  be  re- 
moved ;  and  dikes  of  earth,  usually  termed  levees,  should  be 
raised  on  each  side  of  the  river.  By  increasing  the  water-way  a 
temporary  improvement  only  will  be  effected ;  for,  except  in  the 
season  of  freshets,  the  velocity  of  the  current  at  this  point  will  be 
so  much  decreased  as  to  form  deposites,  which,  at  some  future 
day,  may  prove  a  cause  of  damage.  In  confining  the  water  be- 
tween levees,  two  methods  have  been  tried  ;  the  one  consists  in 


RIVERS. 


34* 


leaving  a  water-way  strictly  necessary  for  the  discharge  of  fresh 
ets  ;  the  other  in  giving  the  stream  a  wide  bed.  The  Po  in  Italj 
and  the  Mississippi  present  examples  of  the  ormer  method  ;  the 
effect  of  whicn  in  both  cases  has  been  to  taise  the  bed  of  the 
stream  so  much  that  in  many  parts  the  water  is  habitually  above 
the  natural  surface  of  the  country,  leaving  it  exposed  to  serioua 
:nundations  should  the  levees  give  way.  The  other  method 
nas  been  tried  on  the  Loire  in  France,  and  observation  has 
proved  that  the  general  level  of  the  bed  has  not  sensibly  risen 
for  a  long  scries  of  years  ;  but  it  has  been  found  that  the  bars, 
which  are  formed  after  each  freshet,  are  shifted  constantly  by 
the  next,  so  that  when  the  waters  have  subsided  to  their  ordinary 
state,  the  navigation  is  extremely  intricate  from  this  cause.  Other 
means  have  been  tried,  such  as  opening  new  channels  at  the  ex- 
posed points,  or  building  dams  above  them  to  keep  the  water 
back ;  but  they  have  all  been  found  to  afford  only  a  temporary 
relief. 

744.  Elbows.  The  constant  wear  of  the  bank,  and  shifting 
of  the  channel  towards  the  cDncave  side  of  elbows,  have  led  to 
various  plans  for  removing  the  inconveniences  which  they  pre- 
sent to  navigation.  The  method  which  has  been  most  generally 
tried  for  this  purpose  consists  in  building  out  dikes,  termed  wing- 
Jems,  from  the  concave  side  into  the  stream,  placing  them  either 
at  right  angles  to  the  thread  of  the  current,  or  obliquely  down 
fttftfun,  so  as  to  deflect  the  current  towards  the  opposite  shore 


Fig.  17+-;RepresentH  a  section  ol  lliu  Limber  whig-dams  on  the  1V>,  formed  of  plank  n.uied 

on  tlie  inclined  pieces  of  the  ribs. 
ab  and  be,  inclined  faces  of  the  dam,  the  first  making  an  angle  of  63°,  and  the  second 

of  £1°  with  the  horizon. 
d  and  e,  pieces  of  the  rib. 
/"and  g,  horizontal  pieces  connecting  the  ribs. 


344  RIVERS. 

Wing-dams  are  usually  constructed  either  of  blocks  of  stone 
)f  crib-work  formed  of  heavy  timbers  filled  in  with  broken  stone 
or  of  alternate  layers  of  gravel  and  fascines.  Within  a  few  years 
back,  wing-dams,  consisting  simply  of  a  series  of  vertical  frames, 
or  ribs,  (Fig.  174,)  strongly  connected  together,  and  covered  on 
the  up-stream  side  by  thick  plank,  which  present  a  broken  in- 
clined plane  to  the  current,  the  lower  part  of  which  is  less  steep 
than  the  upper,  have  been  used  upon  the  Po,  with,  it  is  stated, 
complete  success,  for  arresting  the  wear  of  a  bank  by  the  cur- 
rent. These  dams  are  placed  at  some  distance  above  the  point 
to  be  protected,  and  their  plan  is  slightly  convex  on  the  up-stream 
side. 

Wing-dams  of  the  ordinary  form  and  construction  are  now 
regarded,  from  the  experience  of  a  long  series  of  years  on  the 
Rhine,  and  some  other  rivers  in  Europe,  as  little  serviceable,  if 
not  positively  hurtful,  as  a  river  improvement,  and  the  abandon- 
ment of  their  use  has  been  strongly  urged  by  engineers  in  France. 

The  action  of  the  current  against  the  side  of  the  dam  causes 
whirls  and  counter-currents,  which  are  found  to  undermine  the 
base  of  the  dam,  and  the  bank  adjacent  to  it.  Shallows  and  bars 
are  formed  in  the  bed  of  the  stream,  near  the  dam,  by  the  debris 
borne  along  by  the  current  after  it  passes  the  dam,  giving  very 
frequently  a  more  tortuous  course  to  the  channel  than  it  had  na- 
turally assumed  in  the  elbow.  The  best  method  yet  found  of 
arresting  the  progress  of  an  elbow  is  to  protect  the  concave  bank 
by  a  facing  of  dry  stone,  formed  by  throwing  in  loose  blocks  of 
stone  along  the  foot  of  the  bank,  and  giving  them  the  slope  they 
naturally  assume  when  thus  thrown  in. 

745.  Elbows  upon  most  rivers  finally  reach  that  state  of  de- 
velopment in  which  the  wear  upon  the  concave  side,  from  the 
action  of  the  current,  will  be  entirely  suspended,  and  the  regi- 
men of  the  river  at  these  points  will  remain  stable.  This  state 
will  depend  upon  the  nature  of  the  soil  of  the  banks  and  bed, 
and  the  character  of  the  freshets.  From  observations  made  upon 
the  Rhine,  it  is  stated  that  elbows,  with  a  radius  of  curvature  of 
nearly  3000  yards,  preserve  a  fixed  regimen  ;  and  that  the  banks 
of  those  which  have  a  radius  of  about  1500  yards  are  seldom 
injured  if  properly  faced. 

746.  Attempts  have,  in  some  cases,  been  made  to  shorten  and 
straighten  the  course  of  a  river,  by  cutting  across  the  tongue  of 
land  that  forms  the  convex  bank  of  the  elbow,  and  turning  the 
water  into  a  new  channel.  It  has  generally  been  found  that  the 
stream  in  time  forms  for  itself  a  new  bed  of  nearly  the  same  char 
acter  as  it  originally  had. 

747.  Bars.  To  obtain  a  sufficient  depth  of  water  over  bars, 
the  deposite  must  either  be  scooped  up  by  machinery,  and  b« 


rivers.  345 

conveyed  away,  or  be  removed  by  giving  an  increased  velocity 
to  the  current.  When  the  latter  plan  is  preferred,  an  artificial 
channel  is  formed,  by  contracting  the  natural  way,  confining  it 
between  two  low  dikes,  which  should  rise  only  a  little  above  the 
ordinary  level  of  low  water,  so  that  a  sufficient  outlet  maybe  left 
for  the  water  during  the  season  of  freshets,  by  allowing  it  to  flow 
over  the  dams. 

If  the  river  separates  into  several  channels  at  the  bar,  dams 
should  be  built  across  all  except  the  main  channel,  so  that  by 
throwing  the  whole  of  the  water  into  it  the  effects  of  the  current 
may  be  greater  upon  the  bed. 

The  longitudinal  dikes,  between  which  the  main  channel  is 
confined,  should  be  placed  as  nearly  as  practicable  in  the  direc 
tion  which  the  channel  has  naturally  assumed.  If  it  be  deemed 
advisable  to  change  the  position  of  the  channel,  it  should  be  shift- 
ed to  that  side  of  the  bed  which  will  yield  most  readily  to  the 
action  of  the  current. 

748.  In  situations  where  large  reservoirs  can  be  formed  near 
the  bar,  the  water  from  them  may  be  used  for  removing  it.  Foi 
this  purpose  an  outlet  is  made  from  the  reservoir,  in  the  direction 
of  the  bar,  which  is  closed  by  a  gale  that  turns  upon  a  vertical 
axis,  and  is  so  arranged  that  it  can  be  suddenly  thrown  open  to 
let  off  the  water.  The  chase  of  water  formed  in  this  way  sweep- 
ing over  the  bar  will  prevent  the  accumulation  of  deposites  upon 
it.  This  plan  is  frequently  resorted  to  in  Europe  for  the  removal 
of  deposites  that  accumulate  at  the  mouth  of  harbors  in  those  lo- 
calities where,  from  the  height  to  which  the  tide  rises,  a  great 
head  of  water  can  be  obtained  in  the  reservoirs. 

749.  In  the  improvement  of  the  mouths  of  rivers  which  emptv 
into  the  sea  through  several  channels,  no  obstruction  should  be 
placed  to  the  free  ingress  of  the  tides  through  all  the  channels. 
If  the  main  channel  is  subject  to  obstiuctions  from  deposites. 
dams  should  be  built  across  the  secondary  channels,  which  may 
be  so  arranged  with  cuts  through  them  closed  by  gates,  that  the 
flood-tide  will  meet  with  no  obstruction  from  the  gates,  while  the 
ebb-tide,  causing  the  gates  to  close,  will  be  forced  to  recede 
through  the  main  channel,  which,  in  this  way,  will  be  daily 
scoured,  and  freed  from  deposites  by  the  ebb  current.  The  same 
object  may  be  effected  by  building  dams  without  inlets  across 
the  secondary  channels,  giving  them  such  a  height  that  at  a  cer- 
tain stage  of  the  flood-tide,  the  water  will  flow  over  them,  and 
fill  the  channels  above  the  dams.  The  portion  of  water  thus . 
dammed  in  will  be  forced  through  the  main  channel  at  the  ebb. 

750.  When  the  bed  is  obstructed  by  rocks,  it  may  be  deepened 
try  blasting  the  rocks,  and  removing  the  fragments  with  the  as- 
sistance of  the  diving-bell,  and  other  machinery. 

44 


845  RIVERS. 

751.  In  some  of  our  rivers,  obstructions  of  a  very  dangerous 
character  to  boats  are  met  with,  in  the  trunks  of  large  trees 
which  are  imbedded  in  the  bottom  at  one  end,  while  the  other  is 
near  the  surface  ;  they  are  termed  snags  and  sawyers  by  the 
boatmen.  These  obstructions  have  been  very  successfully  re 
moved,  within  late  years,  by  means  of  machinery,  and  by  pro- 
pelling two  heavy  boats,  moved  by  steam,  which  are  connected 
by  a  ctrong  beam  across  their  bows,  so  that  the  beam  will  strike 
the  snag,  and  either  break  it  off  near  the  bottom,  or  uproot  it 
Other  obstructions,  termed  rafts,  formed  by  the  accumulation  of 
drift  wood  at  points  of  a  river's  course,  are  also  found  in  some 
of  our  western  rivers.  These  are  also  in  process  of  removal,  by 
cutting  through  them  by  various  means  which  have  been  found 
successful. 

752.  Slack-Water  Navigation.  When  the  general  depth  of 
water  in  a  river  is  insufficient  for  the  draught  of  boats  of  the 
most  suitable  size  for  the  trade  on  it,  an  improvement,  termed 
slack-water,  or  lock  and  dam  navigation,  is  resorted  to.  This 
consists  in  dividing  the  course  into  several  suitable  ponds,  by 
forming  dams  to  keep  the  water  in  the  pond  at  a  constant  head  ; 
and  by  passing  from  one  pond  to  another  by  locks  at  the  ends  of 
the  dams. 

753.  The  position  of  the  dams,  and  the  number  requisite,  will 
depend  upon  the  locality.  In  streams  subject  to  heavy  freshets, 
it  will  generally  be  advisable  to  place  the  dams  at  the  widest 
parts  of  the  bed,  to  obtain  the  greatest  outlet  for  the  water  over 
the  dam.  The  dams  may  be  built  either  in  a  straight  line  be- 
tween the  banks  and  perpendicular  to  the  thread  of  the  current, 
or  they  may  be  in  a  straight  line  oblique  to  the  current,  or  their 
plan  may  be  convex,  the  convex  surface  being  up  stream,  or  it 
may  be  a  broken  line  presenting  an  angle  up  stream.  The  three 
last  forms  offer  a  greater  outlet  than  the  first  to  the  water  that 
flows  over  the  darn,  but  are  more  liable  to  cause  injury  to  the 
bed  below  the  stream,  from  the  oblique  direction  which  the  cur- 
rent mj\  receive,  arising  from  the  form  of  the  dam  at  top. 

75  \ .  The  cross  section  of  a  dam  is  usually  trapezoidal,  the 
facj  up-stream  being  inclined,  and  the  one  down-stream  either 
vertical  or  inclined.  When  the  down  stream  face  is  vertical,  the 
velocity  of  the  water  which  flows  over  the  dam  is  destroyed  by 
the  shock  against  the  water  of  the  pond  below  the  dam,  but 
whirls  are  formed  which  are  more  destructive  to  the  bed  than 
would  be  the  action  of  the  current  upon  it  along  the  inclined  face 
of  a  dam.  In  all  cases  the  sides  and  bed  of  the  stream,  for  some 
distance  below  the  dam,  should  be  protected  from  the  action  of 
the  current  by  a  facing  of  dry  stone,  timber,  or  any  other  con- 
struction of  sufficient  durability  for  the  object  in  view. 


RIVERS.  347 

755.  The  dams  should  receive  a  sufficient  height  only  to 
maintain  the  requisite  depth  of  water  in  the  ponds  for  the  pui 
noses  of  navigation.  Any  material  at  hand,  offering  sufficient 
durability  against  the  action  of  the  water,  may  be  resorted  to  in 
iheir  construction.  Dams  of  alternate  layers  of  brush  and  gravel, 
<vith  a  facing  of  plank,  fascines,  or  dry  stone,  answer  very  well 
in  gentle  currents.  If  the  dam  is  exposed  to  heavy  freshets,  to 
3hocks  of  ice,  and  other  heavy  floating  bodies,  as  drift-wood,  it 
would  be  more  prudent  to  form  it  of  dry  stone  entirely,  or  of 
crib-work  filled  with  stone  ;  or,  if  the  last  material  cannot  be  ob- 
tained, of  a  solid  crib- work  alone.  If  the  dam  is  to  be  made 
water-tight,  sand  and  gravel  in  sufficient  quantity  may  be  thrown 
in  against  it  in  the  upper  pond.  The  points  where  the  dam  joins 
the  banks,  which  are  termed  the  roots  of  the  dam,  require  par- 
ticular attention  to  prevent  the  water  from  filtering  around  them. 
The  ordinary  precaution  for  this  is  to  build  the  dam  some  dis 
tance  back  into  the  banks. 

756.  The  safest  means  of  communication  between  the  ponds 
is  by  an  ordinary  lock.  It  should  be  placed  at  one  extremity  of 
the  dam,  an  excavation  in  the  bank  being  made  for  it,  to  secure 
it  from  damage  by  floating  bodies  brought  down  by  the  current. 
The  sides  of  the  lock  and  a  portion  of  the  dam  near  it  should  be 

aised  sufficiently  high  to  prevent  them  from  being  overflowed 
by  the  heaviest  freshets.  When  the  height  to  which  the  freshets 
rise  is  great,  the  leaves  of  the  head  gales  should  be  formed  of 
two  parts,  as  a  single  leaf  would,  from  its  size,  be  too  unwieldy, 
the  lower  portion  being  of  a  suitable  height  for  the  ordinary  man 
oeuvres  of  the  lock  ;  the  upper,  being  used  only  during  the  fresh- 
ets, are  so  arranged  that  their  bottom  cross  pieces  shall  rest, 
when  the  gates  are  closed,  against  the  top  of  the  lower  portions. 
An  arrangement  somewhat  similar  to  this  may  be  made  for  the 
tail  gates,  when  the  lifts  of  the  locks  are  great,  to  avoid  the  diffi 
culty  of  manoeuvring  very  high  gates,  by  permanently  closing 
the  upper  part  of  the  entrance  to  the  lock  at  the  tail  gales,  either 
by  a  wall  built  between  the  side  walls,  or  by  a  permanent  frame- 
work, below  which  a  sufficient  height  is  left  for  the  boats  to  pass. 

757.  A  common,  but  unsafe  method  of  passing  from  one  pond 
to  another,  is  that  which  is  termed  flushing ;  it  consists  of  a 
sluice  in  the  dam,  which  is  opened  and  cldsed  by  means  of  a 
gate  revolving  on  a  vertical  axis,  which  is  so  arranged  that  it  can 
be  manoeuvred  with  ease.  One  plan  for  this  purpose  is  to  divide 
the  gate  into  two  unequal  parts  by  an  axis,  and  to  place  a  valve 
of  such  dimensions  in  the  greater,  that  when  opened  the  surface 
against  which  the  water  presses  shall  be  less  than  that  of  the 
smaller  part.  The  play  of  the  gate  is  thus  rendered  very  simple  ; 
when  the  valve  is  shut,  the  pressure  of  water  on  the  larger  sur* 


348  RIVERS. 

face  closes  it  against  the  sides  of  the  sluice  ;  when  the  v?'ve  is 
opened,  the  gate  swings  round  and  takes  a  position  in  the  dlre^- 
tion  of  the  current.  Various  other  plans  for  flashing,  on  similir 
principles,  are  to  be  met  with. 

758.  When  the  obstruction  in  a  river  cannot  be  overcome  by 
any  of  the  preceding  means,  as  for  example  in  those  considerable 
descents  in  the  bed  known  as  rapids,  where  the  water  acquires 
a  velocity  &o  great  that  a  boat  can  neither  ascend  nor  descend 
with  safety,  resort  must  be  had  to  a  canal  for  the  purpose  of 
uniting  its  navigable  parts  above  and  below  the  obstruction. 

The  general  direction  of  the  canal  will  be  parallel  to  the  bed 
of  the  river.  In  some  cases  it  may  occupy  a  part  of  the  bed  by 
forming  a  dike  in  the  bed  parallel  to  the  bank,  and  sufficiently  far 
from  it  to  give  the  requisite  width  to  the  canal.  Whatever  posi- 
tion the  canal  may  occupy,  every  precaution  should  be  taken  to 
secure  it  from  damage  by  freshets. 

759.  A  lock  will  usually  be  necessary  at  each  extremity  of  the 
canal  where  it  joins  the  river.  The  positions  for  the  extreme  locks 
should  be  carefully  chosen,  so  that  the  boats  can  at  all  times  en- 
ter them  with  ease  and  safety.  The  locks  should  be  secured  by 
guard  gates  and  other  suitable  means  from  freshets ;  and  if  they 
are  liable  to  be  obstructed  by  deposites,  arrangements  should  be 
made  for  their  removal  either  by  a  chase  of  water,  or  by  ma- 
chinery. 

If  the  river  should  not  present  a  sufficient  depth  of  water  at 
all  seasons  for  entering  the  canal  from  it,  a  dam  will  be  required 
at  some  point  near  the  lock  to  obtain  the  depth  requisite. 

It  may  be  advisable  in  some  cases,  instead  of  placing  the  ex- 
treme locks  at  the  outlets  of  the  canal  to  the  river,  to  form  a  ca- 
pacious basin  at  each  extremity  of  the  canal  between  the  lock 
and  river,  where  the  boats  can  lie  in  safety.  The  outlets  from 
the  basins  to  the  rivers  may  either  be  left  open  at  all  times,  or 
else  guard  gates  may  be  placed  at  them  to  shut  off  the  watei 
during  freshets. 


8EAC0AST    IM1-R0VIMENTS.  349 


SEACOAST   IMPROVEMENTS. 

760.  The  following  subdivisions  may  be  made  of  the  works 
belonging  to  this  class  of  improvements.  1st.  Artificial  Road- 
steads. 2d.  The  works  required  for  natural  and  artificial  Har- 
bors. 3d.  The  works  for  the  protection  of  the  seacoast  against 
the  action  of  the  sea. 

761.  Before  adopting  any  definitive  plan  for  the  improvement 
of  the  seacoast  at  any  point,  the  action  of  the  tides,  currents,  and 
waves  at  that  point  must  be  ascertained. 

762.  The  theory  of  tides  is  well  understood  ;  their  rise  and 
duration,  caused  by  the  attraction  of  the  sun  and  moon,  are  also  de- 
pendent on  the  strength  and  direction  of  the  wind,  and  the  confor- 
mation of  the  shore.  Along  our  own  seaboard,  the  highest  tides 
vary  greatly  between  the  most  southern  and  northern  parts.  At 
Eastport,  Me.,  the  highest  tides,  when  not  affected  by  the  wind, 
vary  between  twenty-five  and  thirty  feet  above  the  ordinary  low 
water.  At  Boston  they  rise  from  eleven  to  twelve  feet  above 
the  same  point,  under  similar  circumstances ;  and  from  New- 
York,  following  the  line  of  the  seaboard  to  Florida,  they  seldom 
rise  above  five  feet. 

763.  Currents  are  principally  caused  by  the  tides,  assisted,  in 
some  cases,  by  the  wind.  The  theory  of  their  action  is  simple. 
From  the  main  current,  which  sweeps  along  the  coast,  secondary 
currents  proceed  into  the  bays,  or  indentations,  in  a  line  more  or 
less  direct,  until  they  strike  some  point  of  the  shore,  from  which 
they  are  deflected,  and  frequently  separate  into  several  others, 
the  main  branch  following  the  general  direction  which  it  had 
when  it  struck  the  shore,  and  the  others  not  unfrequently  taking 
an  opposite  direction,  forming  what  are  termed  counter  currents, 
and,  at  points  where  the  opposite  currents  meet,  that  rotary  mo- 
tion of  the  water  known  as  whirlpools.  The  action  of  currents 
on  the  coast  is  to  wear  it  away  at  those  points  against  which 
they  directly  impinge,  and  to  transport  the  debris  to  other  points, 
thus  forming,  and  sometimes  removing,  natural  obstructions  to 
navigation.  These  continual  changes,  caused  by  currents,  make 
it  extremely  difficult  to  foresee  their  effects,  and  to  foretell  the 
consequences  which  will  arise  from  any  change  in  the  direction, 
or  the  intensity  of  a  current,  occasioned  by  artificial  obstacles. 

764.  A  good  theory  of  waves,  which  shall  satisfactorily  ex- 
plain all  their  phenomena,  is  still  a  desideratum  in  science.  It 
is  known  that  they  are  produced  by  vinds  acting  on  the  surface 


350  SEACOAST    IMPROVEMENTS. 

of  the  sea ;  bait  how  far  this  action  extends  below  the  surface 
and  what  are  its  effects  at  various  depths,  are  questions  that  re- 
main to  be  answered.  The  most  commonly  received  theory  is, 
that  a  wave  is  a  simple  oscillation  of  the  water,  in  which  each 
partide  rises  and  falls,  in  a  vertical  line,  a  certain  distance  during 
each  oscillation,  without  receiving  any  motion  of  translation  in  a 
Horizontal  direction.  It  has  been  objected  to  this  theory  that  it 
fails  to  explain  many  phenomena  observed  in  connection  with 
waves. 

In  a  recent  French  work  on  this  subject,  its  author,  Colonel 
Emy,  an  engineer  of  high  standing,  combats  the  received  theory  ; 
and  contends  that  the  particles  of  water  receive  also  a  motion 
of  translation  horizontally,  which,  with  that  of  ascension,  causes 
the  particles  to  assume  an  orbicular  motion,  each  particle  de- 
scribing an  orbit,  which  he  supposes  to  be  elliptical.  He  farther 
contends,  that  in  this  manner  the  particles  at  the  surface  com- 
municate their  motion  to  those  just  below  them,  and  these  again 
to  the  next,  and  so  on  downward,  the  intensity  decreasing  from 
the  surface,  without  however  becoming  insensible  at  even  very 
considerable  depths  ;  and  that,  in  this  way,  owing  to  the  reaction 
from  the  bottom,  an  immense  volume  of  water  is  propelled  along 
the  bottom  itself,  with  a  motion  of  translation  so  powerful  as  to 
overthrow  obstacles  of  the  greatest  strength  if  directly  opposed 
to  it.  From  this  he  argues  that  walls  built  to  resist  the  shock  of 
the  waves  should  receive  a  very  great  batir  at  the  base,  and  that 
this  batir  should  be  gradually  decreased  upward,  until,  towards 
the  top,  the  wall  should  project  over,  thus  presenting  a  concave 
surface  at  top  to  throw  the  water  back.  By  adopting  this  form, 
he  contends  that  the  mass  of  water,  which  is  rolled  forward,  as 
it  were,  on  the  bottom,  when  it  strikes  the  face  of  the  wall,  will 
ascend  along  it,  and  thus  gradually  lose  its  momentum.  These 
views  of  Colonel  Emy  have  been  attacked  by  other  engineers, 
who  have  had  opportunities  to  observe  the  same  phenomena,  on 
the  ground  that  they  are  not  supported  by  facts  ;  and  the  question 
still  remains  undecided.  It  is  certain,  from  experiments  made 
by  the  author  quoted  upon  walls  of  the  form  here  described,  that 
they  seem  to  answer  fully  their  intended  purpose. 

765.  Roadsteads.  The  term  roadstead  is  applied  to  an  in- 
dentation of  the  coast,  where  vessels  may  ride  securely  at  an- 
chor under  all  circumstances  of  weather.  If  the  indentation  is 
covered  by  natural  projections  of  the  land,  or  capes,  from  the 
action  of  the  winds  and  waves,  it  is  said  to  be  land-locked ;  in 
the  contrary  case,  it  is  termed  an  open  roadstead. 

The  anchorage  of  open  roadsteads  is  often  insecure,  owing  to 
violent  winds  setting  into  them  from  the  sea,  and  occasioning 
high  waves,  which,  are  very  straining  to  the  moorings.     The 


SEACOAST   IMPROVEMENTS.  351 

remedy  applied  in  this  case  is  to  place  an  obstruction,  near  the 
entrance  of  the  roadstead,  to  break  the  force  of  the  waves  frotr 
the  sea.  These  obstructions,  termed  breakwaters,  are  artificial 
islands  of  greater  or  less  extent,  and  of  variable  form,  according 
to  the  nature  of  the  case,  made  by  throwing  heavy  blocks  oi 
stone  into  the  sea,  and  allowing  them  to  take  their  own  bed. 

The  first  great  work  of  this  kind  undertaken  in  modern  times, 
was  the  one  at  Cherbourg  in  France,  to  cover  the  roadstead  in 
front  of  that  town.  After  some  trials  to  break  the  effects  of  the 
waves  on  the  roadstead  by  placing  large  conical  shaped  struc- 
tures of  timber  filled  with  stones  across  it,  which  resulted  in 
failure,  as  these  vessels  were  completely  destroyed  by  subsequent 
storms,  the  plan  was  adopted  of  forming  a  breakwater  by  throw- 
ing in  loose  blocks  of  stone,  and  allowing  the  mass  to  assume  the 
form  produced  by  the  action  of  the  waves  upon  its  surface.  The 
subsequent  experience  of  many  years,  during  which  this  work 
has  been  exposed  to  the  most  violent  tempests,  has  shown  that 
the  action  of  the  sea  on  the  exposed  surface  is  not  very  sensible 
at  this  locality  at  a  depth  of  about  20  feet  below  the  water  level 
of  the  lowest  tides,  as  the  blocks  of  stone  forming  this  part 
of  the  breakwater,  some  of  which  do  not  average  over  40  lbs. 
in  weight,  have  not  been  displaced  from  the  slope  the  mass 
first  assumed,  which  was  somewhat  less  than  one  perpendicular 
to  one  base.  From  this  point  upwards,  and  particularly  between 
the  levels  of  high  and  low  water,  the  action  of  the  waves  has 
been  very  powerful  at  times,  during  violent  gales,  displacing 
blocks  of  several  tons  weight,  throwing  them  over  the  top  of  the 
breakwater  upon  the  slope  towards  the  shore.  Wherever  this 
part  of  the  surface  has  been  exposed  the  blocks  of  stone  have 
been  gradually  worn  down  by  the  action  of  the  waves,  and  the 
slope  has  become  less  and  less  steep,  from  year  to  year,  until 
finally  the  surface  assumed  a  slightly  concave  slope,  which,  at 
some  points,  was  as  great  as  ten  base  to  one  perpendicular. 

The  experience  acquired  at  this  work  has  conclusively  shown 
that  breakwaters,  formed  of  the  heaviest  blocks  of  loose  stone, 
are  always  liable  to  damage  in  heavy  gales  when  the  sea  breaks 
over  them,  and  that  the  only  means  of  securing  them  is  by  cov 
ering  the  exposed  surface  with  a  facing  of  heavy  blocks  of  ham 
mered  stone  carefully  set  in  hydraulic  cement. 

As  the  Cherbourg  breakwater  is  intended  also  as  a  military 
construction,  for  the  protection  of  the  roadstead  against  an  ene- 
my's fleet,  the  cross  section  shown  in  (Fig.  175)  has  been  adopt- 
ed for  it.  Profiting  by  the  experience  of  many  years'  observation, 
't  was  decided  to  construct  the  work  that  forms  the  cannon  battery 
of  solid  masonry  laid  on  a  thick  and  broad  bed  of  beton.  The 
top  surface  of  the  breakwater  is  covered  with  heavy  loose  blocks 


355 


SEACOAST  IMPR  JVEMENTS. 


of  stone,  and  the  foot  of  the  wall  on  the  face  is  protected  by 
large  blocks  of  artificial  stone  formed  of  beton.  The  top  of  the 
battery  is  about  12  feet  above  the  highest  water  level. 


Fig.  17.")— Represents  a  section  of  the  Cherbourg  breakwater. 

A,  mass  of  stone. 

B,  battery  of  masonry. 

The  next  work  of  the  kind  was  built  to  cover  the  roadstead  of 
Plymouth  in  England.  Its  cross  section  was,  at  first,  made  with 
an  interior  slope  of  one  and  a  half  base  to  one  perpendicular,  and 
an  exterior  slope  of  only  three  base  to  one  perpendicular ;  but 
from  the  damage  it  sustained  in  the  severe  tempests  in  the 
winter  of  1816-17,  it  is  thought  that  its  exterior  slope  was  too 
abrupt. 

A  work  of  the  same  kind  is  still  in  process  of  construction  on 
our  coast,  off  the  mouth  of  the  Delaware.  The  same  cross  sec- 
tion has  been  adopted  for  it  as  in  the  one  at  Cherbourg. 

All  of  these  works  were  made  in  the  same  way,  discharging 
the  stone  on  the  spot,  from  vessels,  and  allowing  it  to  take  its 
own  bed,  except  for  the  facing,  where,  when  practicable,  the 
blocks  were  carefully  laid,  so  as  to  present  a  uniform  surface  to 
the  waves.  The  interior  of  the  mass,  in  each  case,  has  been 
formed  of  stone  in  small  blocks,  and  the  facing  of  very  large 
blocks.  It  is  thought,  however,  that  it  would  be  more  prudent 
to  form  the  whole  of  large  blocks,  because,  were  the  exterior  to 
suffer  damage,  and  experience  shows  that  the  heaviest  blocks  yet 
used  have  at  times  been  displaced  by  the  shock  of  the  waves,  the 
interior  would  still  present  a  great  obstacle. 

From  the  foregoing  details,  respecting  the  cross  sections  of 
breakwaters,  which  from  experiment  have  been  found  to  answer, 
the  proper  form  and  dimensions  of  the  cross  section  in  similar 
cases  may  be  arranged.  As  to  the  plan  of  such  works,  it  must 
depend  on  the  locality.  The  position  of  the  breakwater  should 
be  chosen  with  regard  to  the  direction  of  the  heaviest  swells  from 
the  sea.  .nto  the  roadstead, — the  action  of  the  current,  and  that 
of  the  waves.  The  part  of  the  roadstead  which  it  covers  should 
afford  a  proper  depth  of  water,  and  secure  anchorage  for  vessels 
of  the  largest  class,  during  the  most  severe  storms ;  and  vessels 
should  be  able  to  double  the  breakwater  under  all  circumstances! 


SEACOAST  IMPROVEMENTS.  353 

of  wind  and  tide.  Such  a  position  should,  moreover,  be  chosen 
that  there  will  be  no  liability  to  obstructions  being  formed  within 
the  roadstead,  or  at  any  of  its  outlets,  from  the  change  in  the 
current  which  may  be  made  by  the  breakwater. 

766.  The  difficulty  of  obtaining  very  heavy  blocks  of  stone, 
u£  well  as  their  great  cost,  has  led  to  the  suggestion  of  substitu 
ting  for  their,  blocks  of  artificial  stone,  formed  of  concrete,  which 
can  be  made  of  any  shape  and  size  desirable.  This  plan  has 
been  tried  with  success  in  several  instances,  particularly  in  a 
jetty  or  mole,  at  Algiers,  constructed  by  the  French  government. 
The  beton  for  a  portion  of  this  work  was  placed  in  large  boxes, 
the  sides  of  which  were  of  wood,  shaped  at  bottom  to  correspond 
to  the  irregularities  of  the  bottom  on  which  the  beton  was  to  be 
spread.  The  bottom  of  the  box  was  made  of  strong  canvass  tar- 
red. These  boxes  were  first  sunk  in  the  position  for  which  they 
were  constructed,  and  then  filled  with  the  beton. 

767.  Harbors.  The  term  harbor  is  applied  to  a  secure  an- 
chorage of  a  more  limited  capacity  than  a  roadstead,  and  there- 
fore offering  a  safer  refuge  during  boisterous  weather.  Harbors 
are  either  natural,  or  artificial. 

768.  An  artificial  harbor  is  usually  formed  by  enclosing  a 
space  on  the  coast  between  two  arms,  or  dikes  of  stone,  or  of 
wood,  termed  jetties,  which  project  into  the  sea  from  the  shore, 
in  such  a  way  as  to  cover  the  harbor  from  the  action  of  the  wind 
and  waves. 

769.  The  plan  of  each  jetty  is  curved,  and  the  space  enclosed 
by  the  two  will  depend  on  the  number  of  vessels  which  it  may 
be  supposed  will  be  in  the  harbor  at  the  same  time.  The  dis- 
tance between  the  ends,  or  heads,  of  the  jetties,  which  forms  the 
mouth  of  the  harbor,  will  also  depend  on  local  circumstances  ; 
it  should  seldom  be  less  than  one  hundred  yards,  and  generally 
need  not  be  more  than  five  hundred.  There  are  certain  winds 
at  every  point  of  a  coast  which  are  more  unfavorable  than  others 
to  vessels  entering  and  quitting  the  harbor,  and  to  the  tranquil- 
lity of  its  water.  One  of  the  jetties  should,  on  this  account,  be 
longer  than  the  other,  and  be  so  placed  that  it  will  both  break 
the  force  of  the  heaviest  swells  from  the  sea  into  the  mouth  of 
the  harbor,  and  facilitate  the  ingress  and  egress  of  vessels,  by 
preventing  them  from  being  driven  by  the  winds  on  the  other 
jetty,  just  as  they  are  entering  or  quitting  the  mouth. 

770.  The  cross  section,  and  construction  of  a  stone  jetty  differ 
in  nothing  from  those  of  a  breakwater,  except  that  the  jetty  is 
usually  wider  on  top,  thirty  feet  being  allowed,  as  it  serves  foi 
a  wharf  in  unloading  vessels.  The  head  of  the  jetty  is  usually 
made  circular,  and  considerably  broader  than  the  other  parts,  as 
:t,  in  some  instances,  receives  a  lighthouse,  and  a  battery  of  ca»- 

45 


854 


SEACOAST  IMPROVEMENTS. 


non.  It  should  be  made  with  great  care,  of  large  blocks  of  stone 
well  united  by  iron,  or  copper  cramps,  and  the  exterior  courses 
should  moreover  be  protected  by  fender  beams  of  heavy  timber, 
to  receive  the  shock  of  floating  bodies. 

771.  Wooden  jetties  are  formed  of  an  open  framework  of 
heavy  timber,  the  sides  of  which  are  covered  on  the  interior  by 
a  strong  sheeting  of  thick  plank.  Each  rib  of  the  frame 
(Fig.  176)  consists  of  two  inclined  pieces,  which  form  the  sides 


Fig.  17G— Represents  a  cross  section  of  a  wooden  jetty. 

a,  foundation  piles. 

b,  inclined  side  pieces. 
.  c,  middle  upright. 

a,  cross  pieces  bolted  in  pairs. 

e,  struts. 

m,  longitudinal  pieces  bolted  in  pairs. 

o,  parapet. 

— of  an  upright  centre  piece, — and  of  horizontal  clamping  pieces, 
which  are  notched  and  bolted  in  pairs  on  the  inclined  and  upright 
pieces ;  the  inclined  pieces  are  farther  strengthened  by  struts, 
which  abut  against  them  and  the  upright.  The  ribs  are  con- 
nected by  large  string-pieces,  laid  horizontally,  which  are  notched 
and  bolted  on  the  inclined  pieces,  the  uprights,  and  the  clamping 
pieces,  at  their  points  of  junction.  The  foundation,  on  which 
this  framework  rests,  consists  usually  of  three  rows  of  large 
piles  driven  under  the  foot  of  the  inclined  pieces  and  the  uprights. 
The  rows  of  piles  are  firmly  connected  by  cross  and  longitudinal 
beams  notched  and  bolted  on  them ;  and  they  are,  moreover, 
firmly  united  to  the  framework  in  a  similar  manner.  The  inte- 
rior sheeting  does  not,  in  all  cases,  extend  the  entire  length  of 
the  sides,  but  open  spaces,  termed  clear-ways,  are  often  left,  t« 


SEACOAST    IMPROVEMENTS.  355 

give  a  free  passage  and  spread  to  the  waves  confined  between 
the  jetties,  for  the  purpose  of  forming  smooth  water  in  the  chan 
ael.  If  the  jetties  are  covered  at  their  back  with  earth,  the  cleai 
ways  receive  the  form  of  inclined  planes. 

The  foundation  of  the  jetties  requires  particular  care,  espe- 
cially when  the  channel  between  them  is  very  narrow.  Loose 
stone  thrown  around  the  piles  is  the  ordinary  construction  used 
for  this  purpose  ;  and,  if  it  be  deemed  necessary,  the  bottom  of 
the  entire  channel  may  be  protected  by  an  apron  of  brush  and 
loose  stone. 

The  top  of  the  jetties  is  covered  with  a  flooring  of  thick  plank, 
which  serves  as  a  wharf.  A  strong  hand  railing  should  be 
placed  on  each  side  of  the  flooring  as  a  protection  against  acci- 
dents. The  sides  of  jetties  have  been  variously  inclined ;  the 
more  usual  inclination  varies  between  three  and  four  perpendicu- 
lar to  one  base. 

772.  Jetties  are  sometimes  built  out  to  form  a  passage  to  a 
natural  harbor,  which  is  either  very  much  exposed,  or  subject  to 
bars  at  its  mouth.  By  narrowing  the  passage  to  the  harbor  be- 
tween the  jetties,  great  velocity  is  given  to  the  current  caused 
by  the  tide,  and  this  alone  will  free  the  greater  part  of  the  chan- 
nel from  deposites.  But  at  the  head  of  the  jetties  a  bar  will,  in 
almost  every  case,  be  found  to  accumulate,  from  the  current 
along  shore,  which  is  broken  by  the  jetties,  and  from  the  dimin- 
ished velocity  of  the  ebbing  tides  at  this  point.  To  remove  these 
bars  resort  may  be  had,  in  localities  where  they  are  left  nearly 
dry  at  low  water,  to  reservoirs,  and  sluices,  arranged  with  turn- 
ing gates,  like  those  adverted  to  for  river  improvements.  The 
reservoirs  are  formed  by  excavating  a  large  basin  in-shore,  at 
some  suitable  point  from  which  the  collected  water  can  be  di- 
rected, with  its  full  force,  on  the  bar.  The  basin  will  be  filled 
at  flood-tide,  and  when  the  ebb  commences  the  sluice  gates  will 
be  kept  closed  until  dead  low  water,  when  they  should  all  be 
opened  at  once  to  give  a  strong  water  chase 

773.  In  harbors  where  vessels  cannot  be  safely  and  conve- 
niently moored  alongside  of  the  quays,  large  basins,  termed  wet- 
docks,  are  formed,  in  which  the  water  can  be  kept  at  a  constant 
level.  A  wet-dock  may  be  made  either  by  an  in-shore  excavation, 
or  by  enclosing  a  part  of  the  harbor  with  strong  water-tight  walls ; 
the  hrst  is  the  more  usual  plan.  The  entrance  to  the  basin  may 
be  by  a  simple  sluice,  closed  by  ordinary  lock  gates,  or  by  meana 
of  an  ordinary  lock.  With  the  first  method  vessels  can  enter 
the  basin  only  at  high  tide  ;  by  the  last  they  may  be  entered  or 
passed  out  at  any  period  of  the  tide.  The  outlet  of  the  lock 
should  be  provided  with  a  pair  of  guard  gates,  to  be  shut  against 
very  high  tides,  or  in  cases  of  danger  from  storms. 


356  SEACOAST    IMPROVEMENTS. 

774.  The  construction  of  the  locks  for  basins  differs  in  nothirg 
in  principle,  from  that  pursued  in  canal  locks.  The  greatest 
care  will  necessarily  be  taken  to  form  a  strong  mass  free  from  all 
danger  of  accidents.  The  gates  of  a  basin-lock  are  made  convex 
towards  the  head  of  water,  to  give  them  more  strength  to  resist 
the  great  pressure  upon  them.  They  are  hung  and  manoeuvred 
differently  from  ordinary  lock  gates  ;  the  quoin-post  is  attached 
to  the  side  walls  in  the  usual  way  :  but  at  the  foot  of  the  mitre- 
post  an  iron  or  brass  roller  is  attached,  which  runs  on  an  iron 
roller  way,  and  thus  supports  that  end  of  the  leaf,  relieving  the 
collar  of  the  quoin-post  from  the  strain  that  would  be  otherwise 
thrown  on  it,  besides  giving  the  leaf  an  easy  play.  Chains  are 
attached  to  each  mitre-post  near  the  centre  of  pressure  of  the 
water,  and  the  gate  is  opened,  or  closed,  by  means  of  windlasses 
to  which  the  other  ends  of  the  chains  are  fastened. 

775.  The  quays  of  wet-docks  are  usually  built  of  masonry. 
Both  brick  and  stone  have  been  used  ;  the  facing  at  least  should 
be  of  dressed  stone.  Large  fender-beams  may  be  attached  to 
the  face  of  the  wall,  to  prevent  it  from  being  brought  in  contact 
with  the  sides  of  the  vessels.  The  cioss  section  of  quay-walls 
should  be  fixed  on  the  same  principles  as  that  of  other  sustaining 
walls.  It  might  be  prudent  to  add  buttresses  to  the  back  of  the 
wall  to  strengthen  it  against  the  shocks  of  the  vessels. 

776.  Quay-walls  with  us  are  ordinarily  made  either  by  form- 
ing a  facing  of  heavy  round  or  square  piles  driven  in  juxtaposition, 
which  are  connected  by  horizontal  pieces,  and  secured  from  the 
pressure  of  the  earth  filled  in  behind  them  by  land-ties ;  or,  by 
placing  the  pieces  horizontally  upon  each  other,  and  securing 
them  by  iron  bolts.  Land-ties  are  used  to  counteract  the  pres- 
sure of  the  earth  or  rubbish  which  is  thrown  in  behind  them  to 
form  the  surface  of  the  quay.  Another  mode  of  construction, 
which  is  found  to  be  strong  and  durable,  is  in  use  in  our  Eastern 
seaports.  It  consists  in  making  a  kind  of  crib-work  of  large 
blocks  of  granite,  and  filling  in  with  earth  and  stone  rubbish. 
The  bottom  course  of  the  crib  may  be  laid  on  the  bed  of  the 
river,  if  it  is  firm  and  horizontal ;  in  the  contrary  case  a  strong 
grillage,  termed  a  cradle,  must  be  made,  and  be  sunk  to  receive 
the  stone  work.  The  top  of  the  cradle  should  be  horizontal,  and 
the  bottom  should  receive  the  same  slope  as  that  of  the  bed,  in 
order  that  when  the  stones  are  laid  they  may  settle  horizontally. 

777.  Dikes.  To  protect  the  lowlands  bordering  the  ocean  from 
inundations,  dikes,  constructed  of  ordinary  earth,  and  faced  to- 
wards the  sea  with  some  material  which  will  resist  the  action  of 
the  current,  are  usually  resorted  to. 

The  Dutch  dikes,  by  means  of  which  a  large  extent  of  country 
has  been  reclaimed  and  protected  from  the  sea,  are  the  most  re 


ZEACOAST  IMPROVEME  TTS  851 

markable  structi  res  of  this  kind  in  existence  The  cross  section 
of  those  dikes  is  of  a  trapezoidal  form,  the  width  at  top  averaging 
from  four  to  six  feet,  the  interior  slope  being  the  same  as  the  na- 
tural slope  of  the  earth,  and  the  exterior  slope  varying,  according 
to  circumstances,  between  three  and  twelve  base  to  one  perpendic- 
ular. The  top  of  the  dike,  for  perfect  safety,  should  be  about 
six  feet  above  the  level  of  the  highest  spring  tides,  although,  in 
many  places,  they  are  only  two  or  three  above  this  level. 

The  earth  for  these  dikes  is  taken  from  a  ditch  in-shore,  be- 
tween which  and  the  foot  of  the  dike  a  space  of  about  twenty 
feet  is  left,  which  answers  for  a  road.  The  exterior  slope  is  va- 
riously faced,  according  to  the  means  at  hand,  and  the  character 
of  the  current  and  waves  at  the  point.  In  some  cases,  a  strong 
straw  thatch  is  put  on,  and  firmly  secured  by  pickets,  or  other 
means  ;  in  others,  a  layer  of  fascines  is  spread  over  the  thatch, 
and  is  strongly  picketed  to  it,  the  ends  of  the  pickets  being  al- 
lowed to  project  out  about  eighteen  inches,  so  that  they  can  re- 
ceive a  wicker-work  formed  by  interlacing  them  with  twigs  ; 
the  spaces  between  this  wicker-work  being  filled  with  broken 
stone  ;  this  forms  a  very  durable  and  strong  facing,  which  resisls 
not  only  the  action  of  the  current,  but,  by  its  elasticity,  the  shocks 
of  the  heaviest  waves. 

The  foot  of  the  exterior  slope  requires  peculiar  care  for  its 
protection  ;  the  shore,  for  this  purpose,  is  in  some  places  cover- 
ed with  a  thick  apron  of  brush  and  gravel  in  alternate  layers,  to 
a  distance  of  one  hundred  yards  into  the  water  from  the  foot  of 
the  slope. 

On  some  parts  of  the  coast  of  France,  where  it  has  been  found 
necessary  to  protect  it  from  encroachments  of  the  sea,  a  cross 
section  has  been  given  to  the  dikes  towards  the  sea,  of  the  same 
form  as  the  one  which  the  shore  naturally  takes  from  the  action  of 
the  waves.  The  dikes  in  other  respects  are  constructed  and  faced 
after  the  manner  which  has  been  so  long  in  practice  in  Holland. 

778.  Groins.  Constructions,  termed  groins,  are  used  when- 
ever it  becomes  necessary  to  check  the  effect  of  the  current 
along  the  shore,  and  cause  deposites  to  be  formed.  These  are 
artificial  ridges  which  rise  a  few  feet  only  above  the  surface  of 
the  beach,  and  are  built  out  in  a  direction  either  perpendiculai 
to  that  of  the  shore,  or  oblique  to  it.  They  are  constructed  ei- 
ther of  clay,  which  is  well  rammed  and  protected  on  the  surface 
by  a  facing  of  fascines  or  stones  ;  or  of  layers  of  fascines  ;  or  of 
one  or  two  rows  of  short  piles  driven  in  juxtaposition ;  or  any 
other  means  that  the  locality  may  furnish  may  be  resorted  to ; 
the  object  being  to  interpose  an  obstacle,  which,  breaking  the 
force  of  the  current,  will  occasion  a  deposite  near  it,  and  thus 
gradually  cause  the  shore  to  gain  upon  the  sea. 


858  SEACOAST    IMPROVEMENTS. 

779.  Sea-walls.  When  the  sea  encroaches  upon  the  land 
forming  a  steep  bluff,  the  face  of  which  is  gradually  worn  away 
a  wall  of  masonry  is  the  only  means  that  will  afford  a  permanent 
protection  against  this  action  of  the  waves.  Walls  made  for  this 
object  are  termed  sea-walls.  The  face  of  a  sea-wall  should  be 
constructed  of  the  most  durable  stone  in  large  blocks.  The 
backing  may  be  of  rubble  or  of  betoi.  The  whole  work  shou.d 
be  laid  with  hydraulic  mortar. 


APPENDIX. 


Note  A  to  Arts.  Framing  and  Bridges. 

Tubular  Frames  of  Wrought  Iron. — Except  for  the  obvious  application  to 
steam  boilers,  sheet  iron  had  not  been  considered  as  suitable  for  structures 
demanding  great  strength,  from  its  apparent  deficiency  in  rigidity ;  and 
although  the  principle  of  gaining  strength  by  a  proper  distribution  of  the 
material,  and  of  giving  any  desirable  rigidity  by  combinations  adapted  to  the 
object  in  view,  were  at  every  moment  acted  upon,  from  the  ever-increasing 
demands  of  the  art,  engineers  seem  not  to  have  looked  upon  sheet  iron  as 
suited  to  such  purposes,  until  an  extraordinary  case  occurred  which  seemed 
about  to  baffle  all  the  means  hitherto  employed.  The  occasion  arose  when  it 
became  a  question  to  throw  a  bridge  of  rigid  material,  for  a  railroad,  across 
the  Menai  Straits;  suspension  systems,  from  their  flexibility,  and  some  actual 
failures,  being,  in  the  opinion  of  the  ablest  European  engineers,  unsuitable  for 
this  kind  of  communication. 

Robert  Stephenson,  who  for  some  years  back  has  held  the  highest  rank 
among  English  engineers,  appears,  from  undisputed  testimony,  to  have  been 
the  first  to  entertain  the  novel  and  bold  idea  of  spanning  the  Strait  by  a  tube 
of  sheet  iron,  supported  on  piers,  of  sufficient  dimensions  for  the  passage 
within  it  of  the  usual  trains  of  railroads.  The  preliminary  experiments  for 
testing  the  practicability  of  this  conception,  and  the  working  out  the  details  of 
its  execution,  were  left  chiefly  in  the  hands  of  Mr.  William  Fairbaim,  to  whom 
the  profession  owes  many  valuable  papers  and  facts  on  professional  topics. 
This  gentleman,  who,  to  a  thorough  acquaintance  with  the  mode  of  conducting 
such  experiments,  united  great  zeal  and  judgment,  carried  through  the  task 
committed  to  him  ;  proceeding  step  by  step,  until  conviction  so  firm  took  the 
place  of  apprehension,  that  he  rejected  all  suggestions  for  the  use  of  any 
auxiliary  means,  and  urged,  from  his  crowning  experiment,  reliance  upon  the 
tube  alone  as  equal  to  the  end  to  be  attained. 

Numerous  experiments  were  made  by  him  upon  tubes  of  circular,  elliptical, 
and  rectangular  cross  section.  The  object  chiefly  kept  in  view  in  these 
experiments  was,  to  determine  the  form  of  cross  section  which,  when  the  tube 


3G0  APPENDIX. 

vas  submitted  to  a  cross  strain,  would  present  an  equality  of  resistance  in  the 
parts  brought  into  compression  and  extension.  It  was  shown,  at  an  early 
stage  of  the  operations,  that  the  circular  ami  elliptical  forms  were  too  weak  in 
Jie  parts  submitted  to  compression,  but  that  the  elliptical  was  the  stronger  of 
he  two;  andthat,  whatever  form  might  be  adopted,  extraordinary  means  would 
be  requisite  to  prevent  the  parts  submitted  to  compression  from  yielding,  by 
"  puckering  "  and  doubling.  To  meet  this  last  difficulty,  the  fortunate  expedient 
was  hit  upon  of  making  the  part  of  the  main  tube,  upon  which  the  strain  of 
compression  was  brought,  of  a  series  of  smaller  tubes,  or  cells  of  a  curved  or 
a  rectangular  cross  section.  The  latter  form  of  section  was  adopted  definitively 
for  the  main  tube,  as  having  yielded  the  most  satisfactory  results  as  to  resist- 
ance ;  and  also  for  the  smaller  tubes,  or  cells,  as  most  easy  of  construction  and 
repair. 

As  a  detail  of  each  of  these  experiments  would  occupy  more  space  than  can 
be  given  in  this  work,  that  alone  of  the  tube  which  gave  results  that  led  to  the 
forms  and  dimensions  adopted  for  the  tubular  bridges  subsequently  constructed, 
will  be  given  in  this  place. 

Model  Tube. — The  total  length  of  the  tube  was  78  ft.  The  distance,  or 
bearing  between  the  points  of  support,  on  which  it  was  placed  to  test  its 
strength,  was  75  ft.  Total  depth  of  the  tube  at  the  middle,  4  ft.  6£  in.  Depth 
at  each  extremity,  4  ft.     Breadth,  2  ft.  8  in. 

The  top  of  the  tube  was  composed  of  a  top  and  bottom  plate,  formed  of 
pieces  of  sheet  iron,  abutting  end  to  end,  and  connected  by  narrow  strips 
riveted  to  them  over  the  joints.  These  plates  were  2  ft.  11 -J-  in.  wide.  They 
were  6£  in.  apart,  and  connected  by  two  vertical  side  plates,  and  five  interior 
division  plates,  with  which  they  were  strongly  joined  by  angle  irons,  riveted 
to  the  division  plates,  and  to  the  top  and  bottom  plates  where  they  joined. 
Each  cell,  between  two  division  plates  and  the  top  and  bottom  plates,  was 
nearly  6  in.  wide.  The  sides  of  the  tube  were  made  of  plates  of  sheet  iron 
similarly  connected ;  their  depth  was  3  ft.  6f  in.  A  strip  of  angle  iron,  bent 
to  a  curved  shape,  and  running  from  the  bottom  of  eaeh  end  of  the  tube  to  the 
top  just  below  the  cellular  part,  was  riveted  to  each  side  to  give  it  stiffness. 
Besides  this,  precautions  were  finally  taken  to  stiffen  the  tube  by  diagonal 
braces  within  it.  The  bottom  of  the  tube  was  formed  of  sheets,  abutting  end 
to  end,  and  secured  to  each  other  like  the  top  plates ;  a  continuous  joint, 
running  the  entire  length  of  the  tube  along  the  centre  line  of  the  bottom,  was 
secured  by  a  continuous  strip  of  iron  on  the  under  side,  riveted  to  the  plates 
on  each  side  of  the  joint.     The  entire  width  of  the  bottom  was  2  ft.  11  in. 

The  sheet  iron  composing  the  top  cellular  portion  was  0*147  in.  thick  ;  that 
of  the  sides  0*099  in.  thick.  The  bottom  of  the  tube  at  the  final  experiments, 
to  a  distance  of  20  ft.  on  each  side  of  the  centre,  was  composed  of  two  thick- 
nesses of  sheet  iron,  each  0*25  in.  thick,  the  joints  being  secured  by  strips 
ab)ve  and  below  them  riveted  to  the  sheets ;  the  remainder,  to  the  end  of  the 
tube,  was  formed  of  sheets  0*156  in.  thick. 

The  total  area  of  sheets  composing  the  top  cellular  portion  was  24*024  in.? 
that  of  the  bottom  plates  at  the  centre  portion,  22*450  in. 


APPENDIX. 


361 


The  general  dimensions  of  the  tube  were  one  sixth  those  of  the  proposed 
structure.     Its  weight  at  the  final  experiment,  13,020  lbs. 

The  experiments,  as  already  stated,  were  conducted  with  a  view  to  ohtain 
an  equality  between  the  resistances  of  the  parts  strained  by  compression  and 
those  extended ;  with  this  object,  at  the  end  of  each  experiment,  the  parts 
torn  asunder  at  the  bottom  were  replaced  by  additional  pieces  of  increased 
strength. 

The  following  table  exhibits  the  results  of  the  final  experiments. 


No.  of  Experiments. 

Weight  in  lbs.               Deflection  in  inches 

1 

20,006 

055 

2 

35,776 

0-78 

3 

48,878 

112 

4 

62,274 

1-48 

5 

77,534 

1-78 

6 

92,299 

212 

7 

103,350 

238 

8 

114,660 

2-70 

9 

132,209 

305 

10 

138,060 

323 

11 

143,742 

3-40 

12 

148,443 

358 

13 

153,027 

3-70 

14 

157,728         .     ' 

378 

15 

161,886 

3-88 

16 

164,741 

3-98 

17 

167,614 

4-10 

18 

171,144 

423 

19 

173,912 

433 

20 

177,088 

4-47 

21 

180,017 

455 

22 

183,779 

462 

23 

186,477 

472 

24 

189,170 

481 

25        .        . 

192,892 

The  tube  broke  with  the  weight  in  the  25th  experiment ;  the  cellular  top 
yielding  by  puckering  at  about  2  ft.  from  the  point  where  the  weight  was 
applied.     The  bottom  and  sides  remained  uninjured. 

The  ultimate  deflection  was  489  in. 


Britannia  Tubular  Bridge. — Nothing  further  than  a  succinct  description  of 
this  marvel  of  engineering  will  be  attempted  here,  and  only  with  a  view  of 
showing  the  arrangement  of  the  parts  for  the  attainment  of  the  proposed  end. 
Tt  differs  in  its  general  structure  from  the  model  tube,  chiefly  in  having  the 
bottom  formed  like  the  top,  of  rectangular  cells,  and  in  the  means  taken  for 
giving  stiffness  to  the  sides. 


3'J2  ATPEKDIX. 

The  total  distance  spanned  by  the  bridge  is  1489  ft.  This  is  livided  into 
four  bays,  the  two  in  the  centre  being  each  460  ft.,  and  the  one  at  each  end 
230  ft.  each. 

The  tube  is  1524  ft.  long.  Its  bearing  on  the  centre  pier  is  45  ft;  that  on 
the  two  intermediate  32  ft. ;  and  that  on  each  abutment  17  ft.  6  in.  The 
height  of  the  tube  at  the  centre  pier  is  30  ft. ;  at  the  intermediate  piers  27  ft. ; 
and  at  the  ends  23  ft.  This  gives  to  the  top  of  the  tube  the  shape  of  9 
parabolic  curve. 


mwAvgi  ,      ,      wrr/rr.viM 


VW      *W 


"offlo  1 

B 


r 


m         ■  ■         m 

c 


&     MMnt  JBMm    A   eH&»  tJPa  liJfta 


Fig.  1 — Represents  a  vertical  cross  section  of  the  Britannia  Bridge. 

A,  interior  of  bridge. 

ll,  cells  of  top  cellular  beam. 

C,  cells  of  bottom  cellular  beam. 

a,  top  plates  of  top  and  bottom  beams. 

6,  bottom  plates  of  top  and  bottom  beams. 

c,  division  plates  of  top  and  bottom  beams. 

d  and  t,  strips  riveted  over  the  joints  of  top  and  bottoir  plate*. 

o,  angle  irons  riveted  to  a,  o,  and  c. 

f,  plates  of  sides  of  the  tube  A. 

«   exterior  T  irons  riveted  over  vertical  joints  of  g. 

i,  interior  T  irons  riveted  over  vertical  joints  of  g,   tnd  bent  at  the  angles  of  A, 

beyond  the  second  cell  of  the  top  beam,  and  be)  mil  the  first  of  the  bottom 
m,  triangular  pieces  on  each  side  of  t,  and  riveted  to  them 


APPENDIX.  363 

The  cellular  top  (Fig.  1)  is  divided  into  eight  cells  B,  by  division  plates  c, 
connected  with  the  top  a,  and  bottom  b,  by  angle  irons  o,  riveted  to  the  plates 
connected.  The  different  sheets  composing  the  plates  a  and  b  abut  end  t« 
end  lengthwise  the  tube ;  and  the  joints  are  secured  by  the  strips  d  and  p4 
riveted  to  the  sheets  by  rivets  that  pass  through  the  interior  angle 
irons. 

The  sheets  of  which  this  portion  is  composed  are  each  6  ft.  long,  and 
1  ft.  9  in.  wide ;  those  at  the  centre  of  the  tube  are  IJths  of  an  inch  thick : 
they  decrease  in  thickness  towards  the  piers,  where  they  are  "tlis  of  an  inch 
thick.  The  division  plates  are  of  the  same  thickness  at  the  centre,  and 
decrease  in  the  same  manner  towards  the  piers.  The  rivets  are  1  in.  thick, 
and  are  placed  3  in.  apart  from  centre  to  centre.  The  cells  are  1  ft.  9  in.  by 
1  ft.  9  in.,  so  as  to  admit  a  man  for  painting  and  repairs. 

The  cellular  bottom  is  divided  into  six  cells  C,  each  of  which  is  2  ft.  4  in. 
wide  by  1  ft.  9  in.  in  height.  To  diminish,  as  far  as  practicable,  the  number 
of  joints,  the  sheets  for  the  sides  of  the  cells  were  made  12  ft.  long.  To  give 
bufficient  strength  to  resist  the  great  tensile  strain,  the  top  and  bottom  plates 
of  this  part  are  composed  of  two  thicknesses  of  sheet  iron,  the  one  layer 
breaking  joint  with  the  other.  The  joints  over  the  division  plates  are  secured 
by  angle  irons  o,  in  the  same  manner  as  in  the  cellular  top.  The  joints 
between  the  sheets  are  secured  by  sheets  2  ft.  8  in.  long  placed  over  them, 
which  are  fastened  by  rivets  that  pass  through  the  triple  thickness  of  sheets 
at  these  points.  The  rivets,  for  attaining  greater  strength  at  these  points,  are 
in  lines  lengthwise  of  the  cell.  The  sheets  forming  the  top  and  bottom  plates 
of  the  cells  are  ".ths  of  an  inch  at  the  centre  of  the  tube,  and  decrease  to  ,'4ths 
at  the  ends.  The  division  plates  are  ,*ths  in  the  middle,  and  j'.ths  at  the  ends 
of  the  tube.     The  rivets  of  the  top  and  bottom  plates  are  lj  in.  in  diameter. 


Fig  2— Represents  a  horizontal  cross  section  of  the  T  irons  and  side  plates. 

D,  cross  section  near  centre  of  bridge. 

K,  cross  section  near  the  piers. 

g,  plates  of  the  sides. 

h,  eiterior  T  irons. 

»,  interior  T  irons. 

The  sides  of  the  tube  (Fig.  2)  between  the  cellular  top  and  bottom  are 
formed  of  sheets  g,  2  ft.  wide ;  the  lengths  of  which  are  so  arranged  that 
mere  are  alternately  three  and  four  plates  in  each  pannel,  the  sheets  of  each 
panuel  abutting  end  to  end,  and  forming  a  contir  uous  vertical  joint  between 
the  adjacent  pannels.  These  vertical  joints  are  secured  by  strips  of  iron, 
h  and  t,  of  the  T  cross   section,  placed  over  each  side  of  the  joint,  and 


364 


> 


APPENDIX. 


flamping  the  sheets  of  the  adjacent  pannels  between  them.  The  T  iront 
within  and  without  are  firmly  riveted  together  with  I  in.  rivets,  placed  at 
3  In.  between  their  centres.  Over  the  joints,  between  the  ends  of  the  sheets 
in  each  pannel,  pieces  of  sheet  iron  are  placed  on  each  side,  and  connected  by 
rivets.  The  sheets  of  the  pannels  at  the  centre  of  the  tube  are  *aths  of  an 
inch  thick;  they  increase  to  Jjjths  to  within  about  10  ft.  of  the  piers,  where 
their  thickness  is  again  increased ;  and  the  T  irons  are  here  also  increased  in 
thickness,  being  composed  of  a  strip  of  thick  sheet  iron,  clamped  between 
strips  of  angle  iron  which  extend  from  the  top  to  the  bottom  of  the  joints. 
Tne  object  of  this  increase  of  thickness,  in  the  pannels  and  T  irons  at  the 
piers,  is  to  give  sufficient  rigidity  and  strength  to  resist  the  crushing  strain  at 
these  points. 

The  T  irons  on  the  interior  are  bent  at  top  and  bottom,  and  extended  as 
far  as  the  third  cell  from  the  sides  at  top,  and  to  the  second  at  bottom.  The 
projecting  rib  of  each  in  the  angles  is  clamped  between  two  pieces,  n,  of  sheet 
iron,  to  which  it  is  secured  by  rivets,  to  give  greater  stiffness  at  the  angles  of 
the  tube. 

The  arrangement  of  the  ordinary  T  irons  and  sheets  of  the  pannels  is 
shown  in  cross  section  by  D,  Fig.  2 ;  and  that  of  the  like  parts  near  the  piers 
by  E,  same  Fig. 

For  the  purpose  of  giving  greater  stiffness  to  the  bottom,  and  to  secure 
fastenings  for  the  wooden  cross  sleepers  that  support  the  longitudinal  beams 
on  which  the  rails  lie,  cross  plates  of  sheet  iron,  half  an  inch  thick,  and  10  in. 
in  depth,  are  laid  on  the  bottom  of  the  tube,  from  side  to  side,  at  every  fourth 
rib  of  the  T  iron,  or  6  ft.  apar.t.  These  cross  plates  are  secured  to  the  bottom 
by  angle  iron,  and  are  riveted  also  to  the  T  iron. 

The  tube  is  firmly  fixed  to  the  central  pier,  but  at  the  intermediate  piers  and 
the  abutments  it  rests  upon  saddles  supported  on  rollers  and  balls,  to  allow 
of  the  play  from  contraction  and  expansion  by  changes  of  temperature. 

The  following  tabular  statements  give  the  details  of  the  dimensions,  weights, 
fee,  of  the  Britannia  Bridge.  . 


Total  length  of  each  tube 

"         "       of  tubes  for  each  line 

Greatest  span  of  bay 

Height  of  tubes  at  the  middle 

"  "  intermediate  piers 

"  "  ends 

Extreme  width  of  tubes 

Number  of  rivets  in  one  tube 

Computed  weight  of  tube  274  ft  long 

"  "  3  tubes  274  ft.  long.. 

"  "  1  tube  472  ft.  long j 

"  "  3  tubes  472  ft.  long \ 

"  "  1  tube  over  pier  32ft.  long 


Feet. 


1524 

3048 

460 

30 

27 

23 

14| 

882,000 


es  A"^Ie    . 
iron.     iron. 


Frames  and  beams . 


Total  weight. 


450 

1350 

965 

2895 

64 

M 


5788 


109 
327 
188 
564 
26 
26 


1240 


Rivet 
iron. 


70 
210 
139 
417 
10 
10 


856 


180 
108 
324 

7 
7 


Cast 
iron 


2000 


Total, 


689 
2067 
1400 
4200 
107 
107 
2000 


2000  10,570 


APPENDIX.  365 


Formula  for  reducing  the  Breaking  Weight  of  Wrought  Iron  Tubes. 

Representing  by  A,  the  total  area  in  inches  of  the  cross  section  of  the  metal, 
a  "    d,  the  total  depth  in  inches  of  the  tube. 

"  "    /,  the  length  in  inches  between  the  points  of  support. 

"  "   C,  a  constant  to  be  determined  by  experiment. 

"  "  W,  the  breaking  weight  in  tons. 

Then  the  relations  between  these  elements,  in  tubes  of  cylindrical,  elliptical 
and  rectangular  cross  section,  will  be  expressed  by  * 

The  mean  value  for  C  for  cylindrical  tubes,  deduced  from  several  experi. 
ments,  was  found  to  be  13*03;  that  for  elliptical  tubes,  15'3;  and  that  for 
rectangular  tubes,  21*5. 


366 


APPENDIX. 


Note  B  to  Art.  Reads. 


Plank-Roads. — A  road  covering,  consisting  of  thick  boards,  or  planks, 
resting  on  longitudinal  beams,  or  sleepers,  and  known  as  Plank-Roads,  has, 
within  the  past  few  years,  been  introduced  among  us;  and  from  its  adaptation 
to  our  uncleared  forest  districts,  its  superior  economy  to  the  ordinary  road 
coverings  in  such  localities,  and  its  intrinsic  merits,  as  fulfilling  the  requisites 
of  a  good  road  covering,  is  rapidly  coming  into  extensive  use  throughout  all 
parts  of  our  country. 

Fig  A 
C 


FifB 


b 

[5 

a 

a 

hf 

S 

Fig.  A— Represents  a  cross-section. 

Fig.  B— A  plan  of  a  plank  road. 

a  a,  board  surface. 

b  b,  sills. 

c,  summer  road. 

d  d,  side  surface  drains. 


The  method  most  generally  adopted  in  constructing  plank-roads  consists  ic 
laying  a  flooring,  or  track,  eight  feet  wide,  composed  of  boards  from  nine  to 
twelve  inches  in  width,  and  three  inches  in  thickness,  which  rest  upon  two 
parallel  rows  of  sleepers,  or  sills,  laid  lengthwise  of  the  road,  and  having  their 
centre  lines  about  four  feet  apart,  or  two  feet  from  the  axis  of  the  road.  Sills 
of  various  sized  scantling  have  been  used,  but  experience  seems  in  favor  of 
scantling  about  twelve  inches  in  width,  four  inches  in  thickness,  and  in  lengths 
of  not  less  than  fifteen  to  twenty  feet.  Sills  of  these  dimensions,  laid  flatwise, 
and  firmly  embedded,  present  a  firm  and  uniform  bearing  to  the  boards,  and 
distribute  the  pressure  they  receive  over  so  great  a  surface,  that,  if  the  soil 
npon  which  they  rest  is  compact  and  kept  well  drained,  there  can  be  but  little 


APPENDIX.  367 

settling  and  displacement  of  the  road  surface,  from  the  usual  loads  passing 
over  it.  The  better  to  «ecure  this  uniform  distribution  of  the  pressure,  the 
sills  of  one  row  are  so  laid  as  to  break  joints  with  the  other;  and  to 
prevent  the  ends  of  the  sills  from  yielding  the  usua.  precaution  is  taken  to 
place  short  sills  at  the  joints,  either  beneath  the  main  sills,  or  on  the  same 
level  with  them. 

The  boards  are  laid  perpendicular  to  the  axis  of  the  road,  experience  having 
shown  that  this  position  is  as  favorable  to  their  wear  and  tear  as  any  other, 
and  is  otherwise  the  most  economical.  Their  ends  are  not  in  an  unbroken 
line,  but  so  arranged  that  the  ends  of  every  three  or  four  project  alternately, 
on  each  side  of  the  axis  of  the  road,  three  or  four  inches  beyond  those  next  to 
them,  for  the  purpose  of  presenting  a  short  shoulder  to  the  wheels  of 
vehicles,  to  facilitate  their  coming  upon  the  plank  surface,  when  from  any 
cause  they  may  have  turned  aside.  On  some  roads  the  boards  have  been 
spiked  to  the  sills;  but  this  is,  at  present,  regarded  as  unnecessary,  the 
stability  of  the  boards  being  best  secured  by  well  packing  the  earth  between 
and  around  the  sills,  so  as  to  present,  with  them,  a  uniform  bearing  surface  to 
the  boards,  and  by  adopting  the  usual  precautions  for  keeping  the  subsoil  well 
drained,  and  preventing  any  accumulation  of  rain  water  on  the  surface. 

The  boards  for  plank-roads  should  be  selected  from  timber  free  from  the 
usual  defects,  such  as  knots,  shakes,  &c,  which  would  render  it  unsuitable 
for  ordinary  building  purposes ;  as  durability  is  an  essential  element  in  the 
economy  of  this  class  of  structures.  So  far  as  experience  has  furnished  data, 
boards  of  three  inches  in  thickness  offer  all  the  requisites  of  strength  and 
durability  that  can  be  obtained  from  timber  in  its  ordinary  state,  in  which  it  is 
used  for  plank-roads. 

Besides  the  wooden  track  of  eight  feet,  an  earthen  track  of  twelve  feet  in 
width  is  made,  which  serves  as  a  summer  road  for  light  vehicles,  and  as  a  turn 
out  for  loaded  ones ;  this,  with  the  wooden  track,  gives  a  clear  road  surface  of 
twenty  feet,  the  least  that  can  be  well  allowed  for  a  frequented  road.  It  is 
recommended  to  lay  the  wooden  track  on  the  right  hand  side  of  the  approach 
of  a  road  to  a  town,  or  village,  for  the  proper  convenience  of  the  rural  traffic, 
as  the  heavy  trade  is  to  the  town.  The  surface  of  this  track  receives  a  cross 
slope  from  the  side  towards  the  axis  of  the  road  outwards  of  1  in  32.  The 
surface  of  the  summer  road  receives  a  cross  slope  in  the  opposite  direction  of 
1  in  16.  These  slopes  are  given  for  the  purpose  of  facilitating  a  rapid  surface 
drainage.  The  side  drains  are  placed  for  this  purpose  parallel  to  the  axis  of 
'he  road,  and  connected  with  the  road  surface  in  a  suitable  slope. 

Where,  from  the  character  of  the  soil,  good  summer  roads  cannot  be  had, 
it  will  be  necessary  to  make  wooden  turn  outs,  from  space  to  space,  to 
prevent  the  inconvenience  and  delay  of  miry  roads.  This  it  is  proposed  to 
do  by  laying,  at  these  points,  a  wooden  track  of  double  width,  to  enable 
vehicles  meeting  to  pass  each  otter.  It  is  recommended  to  lay  these  turn 
outs  on  four  or  five  sills,  to  spring  the  boards  slightly  at  the  centre,  and  spike 
their  ends  to  the  exterior  sills. 

The  angle  of  repose,  by  which  the  grade  of  plank-roads  should  be  regu- 


368  AFPENDIX. 

lated,  has  not  yet  been  determined  by  experiment ;  but  as  the  wooden  surface 
is  covered  with  a  layer  of  clean  sand,  fine  gravel,  or  tan  bark,  before  it  is 
thrown  open  to  vehicles,  and  as  it  in  time  becomes  covered  with  a  permanent 
stratum  of  dust,  &c,  it  is  probable  that  this  angle  will  not  materially  diffei 
from  that  on  a  road  with  a  broken  stone  surface,  like  the  one  of  McAdam,  Or 
of  Telford,  when  kept  in  a  thorough  state  of  repair. 

In  some  of  the  earlier  plank-roads  made  in  Canada,  a  width  of  sixteen  feet 
was  given  to  the  wooden  track,  the  boards  of  which  were  laid  upon  four  or 
five  rows  of  sills ;  experience  soon  demonstrated  that  this  was  by  no  means 
an  economical  plan,  as  it  was  found  that  vehicles  kept  the  centre  of  the  wooden 
surface,  which  was  soon  worn  into  a  beaten  track,  whilst  the  remainder  was 
but  slightly  impaired.  This  led  to  the  abandonment  of  the  wide  track  for  the 
one  now  usually  adopted,  which  answers  all  the  ends  of  the  wants  of  travel,  and 
is  much  more  economical,  both  in  the  first  outlay  and  for  subsequent  renewals. 

The  great  advantages  of  plank-roads  over  every  other  kind,  in  a  densely 
wooded  country,  for  the  rural  traffic,  are  so  obvious,  that,  did  not  experience 
teach  us  by  what  mere  accidents,  apparently,  improvements  of  the  most 
important  kind  have  been  suggested  and  carried  into  effect,  it  might  be  a 
subject  of  astonishment  that  they  had  not  been  among  the  first  to  be  intro- 
duced, after  a  trial  of  the  old  corduroy  road,  so  generally  resorted  to  in  the 
early  stages  of  road-making  in  this  country. 


APPENDIX.  300 


Note  C  to  Arts.  441  and  442. 


Methods  of  describing  Curves  composed  of  Arcs  of  Circles. — Tl  *  span  and 
rise  of  an  arch  being  given,  together  with  the  directions  of  the  tangents  to  the 
curve  at  the  springing  lines  and  crown,  an  infinite  numoer  of  curves,  composed 
of  arcs  of  circles,  can  be  determined,  which  shall  sati>fy  the  conditions  of  form- 
ing a  continuous  curve,  or  one  in  which  the  arcs  shall  be  consecutively 
tangent  to  each  other,  and  such  that  those  at  the  springing  lines  and  the 
erown  shall  be  tangent  to  the  assumed  directions  of  the  tangents  to  the  curve 
at  those  points.  To  give  a  determinate  character  to  the  problem,  in  each 
particular  case,  certain  other  conditions  must  be  imposed,  upon  which  the 
solution  will  depend. 

When  the  tangents  to  the  curve  at  the  springing  lines  and  crown  are 
respectively  perpendicular  to  the  span  and  rise,  the  curve  satisfying  the  above 
general  conditions  will  belong  to  the  class  of  oval  or  basket-handle  curves; 
wken  the  tangents  at  the  springing  lines  are  perpendicular  to  the  span,  and 
those  at  the  crown  are  oblique  to  the  rise,  the  curves  will  belong  to  the  class 
of  pointed  or  obtuse  curves. 

In  the  class  of  ovals,  when  the  rise  is  not  less  than  one  third  of  the  span, 
the  oval  of  three  centres  will  generally  give  a  curve  of  a  more  pleasing  form 
to  the  eye  than  one  of  a  greater  number  of  centres ;  when  the  rise  is  less  than 
a  third  of  the  span,  a  curve  of  five,  seven,  or  a  greater  odd  number  of  centres 
will  give,  under  this  point  of  view,  a  more  satisfactory  solution.  In  the 
pointed  and  obtuse  curves  the  number  of  centres  is  even,  and  is  usually 
restricted  to  four. 

Three  Centre  Curves.  To  obtain  a  determinate  solution  in  this  case  it  will 
be  necessary  to  impose  one  more  condition,  which  shall  be  compatible  with 
the  two  general  ones  of  having  the  directions  of  the  tangents  at  the  springing 
lines  and  crown  fixed.  One  of  the  most  simple,  and  at  the  same  time 
admitting  of  a  greater  variety  of  curves  to  choose  from,  is  to  assume  the  radius 
of  the  curve  at  the  springing  lines.  In  order  that  this  condition  shail  be  com- 
patible with  the  other  two,  the  length  assumed  for  this  must  lie  between  zero 
and  the  rise  of  the  arch  ;  for  were  it  zero  there  would  be  but  one  centre,  and 
If  taken  equal  to  the  rise  the  radius  of  the  curve  at  the  crown  would  be 
infinite. 

Let  A  D  (fig.  A)  be  the  half  span,  and  A  C  the  rise.  Having  prolonged  C  A 
indefinitely,  take  any  distance  less  than  A  C,  and  set  it  r  fffrom  D  to  R,  along 
AD;  and  from  C  to  P,  along  A  C.  Join  R  and  P,  which  distance  bisect  by  a 
perpendicular.  Prolong  the  perpendicular,  to  intersect  the  indefinite  prolong- 
ation of  C  A.  Through  this  point  of  intersection  S,  and  the  point  R,  draw  an 
indefinite  line.  From  R,  as  a  centre,  with  the  radius  R  D,  describe  an  arc, 
which  prolong  to  Q  to  intersect  S  R  prolonged.  From  S,  as  a  centre,  with  the 
radius  S  Q,  describe  an  arc,  which,  from  the  construction,  must  pass  through 


870  APPENDIX. 

the  point  C,  and  be  tangent  to  the  first  arc  at  Q  The  centres  R  and  S,  thus 
determined,  and  the  curve  D  D  C  deduced  from  them,  will  satisfy  the  imposed 
conditions. 


Fig.  A. 

The  two  following  constructions,  from  their  simplicity  and  the  agreeable 
form  of  curve  which  they  produce,  ape  in  frequent  use.  The  first  consists  in 
imposing  the  condition  that  each  of  the  three  arcs  shall  be  of  60°;  the  second, 

R 

that  the  ratio  —  between  the  radii  of  the  arcs  at  the  crown  and  springing  line 

shall  be  a  minimum. 

To  construct  the  curve  satisfying  the  former  condition,  let  A  B  be  the 
half  span,  and  A  C  the  rise.  With  the  radius  A  B  describe  A  a  of  90°, — set 
off  on  it  B  b  =  60°, — draw  the  lines  a  b,  b  B  and  A  b, — from  C  draw  a 
parallel  to  a  b,  and  mark  its  intersection  c  with  b  B, — from  c  draw  a  parallel  to 
A  b,  and  mark  its  intersections  iV  and  O  with  A  B,  and  C  A  prolonged.  From 
JV  with  the  radius  N  B  describe  the  arc  B  c, — from  O  with  the  radius  O  c 
describe  the  arc  C  c.  The  curve  B  c  C  will  be  the  half  of  the  one  satisfying 
the  given  conditions  ;  and  N  and  O  two  of  the  centres. 

To  construct  the  curve  satisfying  the  second  condition,  or  d  I  —  )  =  o.  Let 

dr 
A  D  be  the  half  span, — A  C  the  rise.  Draw  D  C,  and  from  C  set  off  on  it 
C  d  =  C  a,  equal  to  the  difference  between  the  half  span  and  rise.  Bisect 
the  distance  D  d  by  a  perpendicular,  which  prolong  to  intersect  D  A,  and 
C  A  prolonged,  at  R  and  S, — from  these  points,  as  centres,  with  the  radii 
R  D  and  S  Q,  describe  the  arcs  D  Q  and  Q  C ;  and  the  curve  D  Q  C  will 
be  the  half  of  the  one  required. 

The  analysis,  from  which  the  ab«  ve  result  is  obtained,  is  of  a  very  simple 
character ;  for  designating  by  R  =  S  C  the  greater  radius,  by  r  =  R  D  the 


APPENDIX. 


311 


esser, — by  a  =  A  D  the  half  span,  and  by  b  =  A  C  the  rise,  there  results, 
from  the  right  angled  triangle  SAR, 

SR,  =  ASt  +  AR\ 
or 

(*  —  r)*«(Jt~.  *)>+(«-*)*, 

from  which  is  obtained 

R       o«  f  ©•    -  2ar 


DifferentU*i.T.£    this 


r  (26  — 

expression,    and 


2r)r 
placing 


its    nrst    differential    co» 


<f) 


efficient  equal  to  zero,  or  making  — : —  =  0,  there  results,  after  the  terms 

ar 

are  reduced, 

a*  +  b*  —  (a 


b)  yV  +  b*_  yV  +  b1 


^  +  y_(._i) 

2a  a         v  2 

^a3  -f  A"J  =  DC,  and  v^a2  +  6*  —  (a  —  6)  =  Dd,  hence  the  given  construc- 
tion for  the  centres  required. 

By  comparing  the  two  methods  just  explained,  for  the  same  span  and  rise, 
it  will  be  seen  that  the  former  gives  a  curve  in  which  the  lengths  of  the  arcs 
differ  less  than  in  the  latter,  and  which  is  therefore  more  agreeable  to  the 
eye. 

Obtuse  and  Pointed  Curves  of  Four 
Centres.  —  Let  A  J?  be  the  half 
span, — A  C  the  rise  of  the  required 
curve, — and  C  D  ihe  direction  of  the 
tangent  to  it  at  the  crown.  At  C 
draw  a  perpendicular  to  C  D.  Take 
any  point  R  on  A  B,  such  that  R  B 
shall  be  less  than  the  perpendicular 
R  6,  from  R  upon  the  tangent  C  D. 
From  C,  on  the  perpendicular  to 
C  D,  set  off  C  d,  equal  to  the 
assumed  distance  R  B, — draw  R  d, 
and  bisect  it  by  a  perpendicular, — 
which  prolong  to  intersect  the  one 
from  C  at  the  point  S, — through  S 
and  R  draw  a  line, — from  R,  with  the 
radius  R  B,  describe  an  arc,  which 
prolong  to  Q,  to  intersect  the  line 
through  S  and  R, — from  S  with  the 
F)_  ^  radius  S  Q,  describe  an  arc,  which 

will  be  tangent  to  the  first  at  Q,  and  pass  through  C.    The  curve  B  Q  C 
will  be  the  half  of  the  one  required  to  satisfy  the  given  conditions. 
The  analogy  between  this  construction  and  the  one  first  given  for  three 


372 


APPENDIX. 


centre  curves  will  be  readily  seen  by  comparing  the  constructions  for  the  two 
Five  Centre  Oval  Curves,  cj*c.  When  the  rise  is  less  than  one  third  of  the 
span,  it  is  found  that  oval  curves  of  a  pleasing  shape  cannot  be  obtained  by 
using  only  three  centres,  and  five,  or  a  greater  odd  number  of  centres  must  bo 
resorted  to.  Besides  the  two  general  conditions  common  to  all  ovals,  a 
greater  number  of  particular  ones  must  be  imposed,  as  the  number  of  centres 
is  increased,  restricting  them  within  the  limits  of  compatibility  with  each  other 
and  with  the  two  common  to  all.  By  imposing,  for  example,  on  the  oval  of 
five  centres,  the  conditions  that  the  radii  of  the  two  consecutive  arcs  from  the 
springing  line  shall  be  assumed  as  the  particular  conditions,  a  very  simple 
construction,  analogous  to  the  one  for  ovals  of  thiee  centres,  will  show  the 
limits  within  which  these  must  be  restricted,  not  to  interfere  with  the  others 
that  are  common  to  all.  Without  stopping  to  illustrate  this  by  an  example, 
which  will  present  no  difficulty  to  any  one  tolerably  conversant  with  the 
elements  of  geometry  to  make  out  alone, «  more  general  rcethod  will  be  given, 
applicable  alike  to  all  curves  of  this  class. 

The  half  span  and  rise  being  given,  let  it 
be  required  to  determine  an  oval  of  five 
centres  with  the  particular  conditions,  that 
the  radii  of  the  consecutive  arcs,  from  the 
springing  line  towards  the  crown,  shall  be 
in  an  increasing  geometrical  progression, — 
in  which  case  the  curvatures  of  the  arcs 
will  be  in  a  decreasing  geometrical  pro- 
gression— and  the  lengths  of  the  consecutive 
arcs  shall  increase  in  a  given  ratio.  Desig- 
nate the  half  span  AB  by  p  (Fig.  C),  the 
rise  by  q, — the  ratio  of  the  radii  by  m, — 
the  ratio  of  the  arcs  by  n,  and  the  number 
of  degrees  in  the  arc  at  the  springing  line 
by  a.  Suppose  the.  centres  O,  P  and  Q 
found,  and  draw  PS  perpendicular  to  AB, 
— and  PR  perpendicular  to  BC  produced. 

The  radii  OA,  PE  and  QD  will  be  re- 
presented respectively  by  r,  rm,  and  rm9, — 
and  the  angles  AOE,  EPD,  and  DQC, 
between  them  by 

n  n* 

a,  a—,  and  a—;  — 

m  ml 

now,  from  the  properties  of  the  figure,  the 
following  equations  are  obtained 


Ftg.  C. 


«  n 

a  +  a  —  +  a  —  =90c 

m  77i- 


(A) 


AB=V  =  r  +  OS+PR    .         .         (B) 
BC  =  q  =  -m,—  (PS+QR),      .         (C) 


APPENDIX.  276 

From  the  right  angle  triangle  OPS,  and  PQR,  there  results, 
OS  =  OP  cos.  a  =  (rm  —  r)  cos.  a  ; 
PS  =  OP  sin.  a  =  (rm  — r)  sin.  a ; 

n  n 

PR  =  PQ  cos.  (a  +  a (-  (rm2  —  rm)  cos.  (a  -f-  a — )  : 

m  3i 

QRssPQ  sin.  (a  +  a —  )  =  (rm* —  rm)  sin.  la  +  a — I  ; 
by  substituting  these  values  in  equations  (B)  and  (C),  there  results, 
p  =  r  X  l-f-(m  —  1)  cos.  a  +  (ma  —  m)  cos.  f )  a\  (^ 

a  =  r  <  rra*  —  (to  —  1)  sin.  a  —  (ma  —  m)  sin.  ( )  a  t '        ^ 

and  by  reduction,  equation  (A)  becomes, 

(m  —  n)  m* 


90°; (G) 


The  equations  (£),  (F)  and  (G)  express,  therefore,  the  relations  which  sub- 
sist between  the  six  quantities  p,  q,  r,  a,  m  and  n  when  the  imposed  conditions 
are  satisfied.  Let  three  of  these  quantities  as  m,  n  and  r  be  assumed,  the 
others  will  be  found  from  the  three  equations  in  question ;  that  is  the  span, 
rise,  and  number  of  degrees  in  the  arc  at  the  springing  line,  which  correspond 
to  the  given  values. 

From  the  solution  here  given,  the  ratio  of  p  to  q  or  —  is  found  ;  but  as  the 

p 
rise  and  span  are  usually  a  part  of  the  data,  this  ratio  —  may     be     different 

from  that  —  of  the  given  half  span  b,  and  rise  c ;  in  which  case  it  will  be 
c 

necessary  to  assume  new  values  for  the  quantities  m,  n,  and  r,  and  find  the 

p 
corresponding  values  of  p,  q,  and  a,  until  the  ratio  —  is  equal  to,  or  nearly  the 

same  as  — .     When  a  suitable  approximation  has  been  obtained,  it  will  be 

easy  to  find  a  curve  which  shall  differ  but  little  from  the  required  one,  and 

b 
whose  half  span  and  rise  shall  have  the  required  ratio  — . 

To  effect  this,  let  x  be  the  quantity  which  must  be  added#  to  p  and  y 
respectively,  to  make  their  ratio  u.e  same  as  that  of  b  to  c ;  this  condition  will 
be  expressed  by  the  equation, 


374  APPENDIX. 

h    p  +  X 

c        q  +  *' 
from  which  there  results 

pc  —  qb 

*~T=T; (H- 

it'  now  this  quantity  be  set  off  from  A  to  M  (Fig.  C),  and  from  C  to  N,  and 
a  new  curve  AN  be  described  from  the  same  centres  O,  P  and  Q,  it  will  be 
parallel  to  the  curve  AC,  whose  half  span  and  rise  are  p  and  q,  and  the  halt 
span  BM,  and  rise  BN,  will  have  the  same  ratio  as  b  to  c.  To  pass  from  this 
curve  to  a  similar  one,  described  on  the  given  half  span  b,  and  rise  c,  it  will  be 
only  necessary  to  multiply  each  line  of  the  figure  QPOMN  by  the  ratio 

b 

p+x' 
or,  substituting  for  x  its  value,  as  determined  in  equation  (H),  by 

b—c 

p—q' 
since  the  figures  being  similar,  their  homologous  lines  are  proportional,  or,  for 
example, 

p  +  x:b::OM:-^—OM', 
p+x 

which  will  give  the  line,  corresponding  to  OM,  in  the  figure  of  which  b  is  the 
half  span,  and  c  the  rise. 

The  method  here  explained  may  be  applied  to  any  number  of  centres,  but 
where  the  rise  is  less  than  one-fourth  of  the  span,  an  oval  of  five  centres  will 
be  found  to  answer  fully  all  the  required  conditions. 

There  are  other  methods  of  describing  the  oval  of  five,  or  a  greater  number 
of  centres,  which  are  rather  more  simple  for  calculation  than  the  general 
method  just  given. 

By  assuming,  for  example,  the  greatest  and  smallest  radii  within  suitable 
limits,  the  intermediate  radius  may  receive  the  condition  of  being  a  mean 
proportional  between  these  two ;  or  designating  it  by  x,  there  will  result 
*  =  ^RXr;  — R  being  the  greatest  radius,  and  r  the  least.  Having  found  x, 
the  position  of  the  intermediate  centre  P  is  found,  by  describing  an  arc  from 
Q  with  a  radius  R — x,  and  another  from  O  with  a  radius  x — r,  and  taking 
their  point  of  intersection  P. 

A  similar  process  might  be  followed  for  an  oval  of  seven  centres,  by  finding 
the  two  intermediate  terms  of  a  geometrt al  progression,  of  which  r  and  R  ore 
the  two  extremes. 


APPENDIX.  875 


Note  D  to  Arts.  270,  cf-c,  on  the  Strengh  of  Materials. 

FlaKici'.y  and  Cohesion,  and  (heir  Measure. — To  arrive  at  somewhat  definite 
notions  on  the  subjects  of  the  elasticity,  and  the  tenacity  or  cohesion  of  solid 
bodies,  and  the  measure  of  the  resistance  which  they  respectively  offer  to  any 
extraneous  force  that  tends  to  disturb  the  natural  state  of  equilibrium  of  the 
elementary  particles  of  which  these  bodies  are  composed,  and  thereby  call  into 
play  their  elasticity  and  cohesion,  it  may  be  well  to  give  a  brief  summary  of 
the  hypotheses  now  most  generally  received  among  writers  on  physics  upon 
these  points;  and  to  show  in  what  manner  the  relations  between  the  dis- 
turbing forces  and  their  effects  can  be  represented  geometrically. 

The  hypotheses  now  generally  admitted  are  that  bodies  are  composed  of 
elementary  molecules,  each  of  which  consists  of  atoms  grouped  together  in 
definite  numbers  or  proportions  according  to  simple  and  regular  natural  laws; 
that  the  positions  of  the  atoms  of  any  molecule,  with  respect  to  each  other, 
cannot  be  so  fur  displaced  by  any  ordinary  extraneous  force  as  to  modify 
their  arrangement,  external  form,  or  mechanical  properties  ;  that  the  element- 
ary molecules,  separated  from  each  other  by  greater  or  lesser  distances,  as 
compared  with  their  own  dimensions,  exert  a  reciprocal  attraction,  which  varies 
in  intensity  not  only  with  their  distances  apart,  but  also  with  their  relative 
positions,  and  that  the  tendency  of  this  reciprocal  attraction  is  to  cause  the 
molecules  to  group  themselves  according  to  regular  laws ;  that,  in  the  natural 
state  of  equilibrium  of  the  molecules,  the  forces  of  attraction  are  balanced  by 
the  forces  of  repulsion  of  the  interposed  caloric,  but  that  these  forces  may  be 
brought  into  play  by  any  extraneous  force  which  tends  to  disturb  the  natural 
state  of  equilibrium  of  the  molecules,  either  by  its  tendency  to  separate  them, 
or  to  press  them  together,  until  they  have  assumed  a  new  position  of  stable 
equilibrium  under  the  combined  effect  of  all  the  forces  in  action. 

To  account  for  the  state  of  equilibrium  of  the  molecules  under  the  action  of 
the  forces  of  attraction  and  repulsion  alone,  it  is  supposed  that  the  atoms  of 
caloric  repel  each  other  at  all  distances,  but  with  a  force  that  decreases  very 
rapidly  in  intensity  as  the  distance  between  the  molecules  increases ; — that  the 
atoms  of  caloric  on  the  contrary  are  attracted  by  the  molecules  of  different 
bodies  with  varying  intensities,  and  which  causes  them  to  collect  around  these 
molecules,  so  as  to  surround  each  one  with  a  kind  of  atmosphere  of  caloric 
that  decreases  in  density  from  the  centre  outwards,  until  it  attains  an  intensity 
equal  to  that  of  the  surrounding  medium; — that,  when  two  molecules  of  a 
body  are  brought  so  near  each  other  as  to  be  within  the  sphere  of  these  forces 
of  attraction  and  repulsion,  the  force  by  which  they  are  repelled  is  simply  that 
between  the  atoms  of  caloric,  whilst  the  one  by  which  they  are  attracted  to 
each  other  is  composed  of  the  mutual  attraction  of  their  matter,  and  of  the 
attraction  of  the  atoms  of  the  caloric  composing  the  atmosphere  of  the  one  for 
the  matter  of  the  other ; — finally,  that  the  intensities  of  these  forces  of  attrao- 


3V6 


APPENDIX. 


tion  and  repulsion  decrease  very  rapidly  as  the  distance  between  the 
molecules  is  increased,  and  so  as  to  become  null,  or  insensible  at  distances 
which  are  appreciable  by  the  senses. 

Admitting  then  the  hypothesis,  that,  in  the  ordinary  state  of  equilibrium  of 
solid  bodies,  the  molecules  are  preserved  in  their  relative  positions  by  a  force 
of  attraction  and  one  of  repulsion  which  balance,  or  destroy  each  other,  and 
that  when  the  distance  at  which  they  are  kept  apart  in  this  state  is  either 
increased  or  decreased  by  an  extraneous  force,  acting  in  the  line  o/  direction 
between  their  centres,  the  force  of  attraction,  or  that  of  repulsion  will  be 
called  into  action,  the  one  or  the  other  being  in  excess,  as  the  extraneous  force 
tends  to  separate  or  press  together  the  molecules,  the  extraneous  force  itself 
measuring  this  excess,  it  will  be  easy  to  represent  these  conditions  of  equili- 
brium generally,  and  to  express  the  law  by  which  these  various  forces  are 
connected. 


Note.— The  points  of  intersection  of  the  ordinate! 
and  the  curve  are  marked  thus  ° ;  those  of  Um 
ordinates  and  tangents  thus  • 


X 


x  curie       x 


To  do  this  let  A  X,  and  A  Y  (Fig.  A),  be  taken  as  the  rectangular  axes  of  a 
eurve  y,  y',  y",  &c.,  the  abscissas  of  which,  as  Ax,  Ax',  Ax",  &c,  represent 


APPENDIX.  377 

the  respective  distances  apart  ,of  two  molecules,  and  the  corresponding  ordin- 
ates  x  y,  x'  y',  x"  y",  &c,  the  corresponding  values  of  the  forces  of  repulsion, 
and  let  z,  z',  z",  &c,  be  a  second  curve  having  the  same  abscissas  as  the  first, 
and  for  its  ordinates  x  z,  x'  z',  x  '  z",  &c,  which  represent  the  values  of  the 
forces  of  attraction  corresponding  to  the  distances  apart  of  the  molecules,  Ax, 
\x"  &c.  These  two  curves  should  intersect  at  some  point,  as  b,  which 
rorresponds  to  the  natural  state  of  equilibrium  of  the  two  molecules,  in  which 
their  distance  apart  is  A  a,  and  the  forces  of  repulsion  and  attraction,  repre- 
sented by  the  ordinate  a  b,  are  equal.  From  the  common  point  b,  towards  the 
axis  AY,  the  two  curves  should  approach  rapidly  this  axis,  without  however 
ever  meeting  it,  since  matter  is  impenetrable,  and  in  this  part  the  forces  of 
repulsion  represented  by  the  ordinates  x  y,  &c,  will  be  greater  than  those  of 
attraction,  represented  by  the  ordinates  x  z,  &c.  To  the  right  of  the  point  b, 
the  curves  will  recede  from  each  other  making  the  ordinates  x  z,  &c.,  greater 
than  the  corresponding  ones  x  y,  &c,  until  some  distance  Ax"  between  the 
nolecules  is  reached,  when  the  difference  z"  y  between  the  corresponding 
.rdinates  is  a  maximum,  and  from  which  point  the  curves  will  again  approach 
<ach  other,  to  intersect  at  a  second  point  &',  having  the  common  ordinate  a'  b'y 
^here  the  forces  are  again  in  equilibrium,  and  beyond  wbich,  to  the  right,  that 
.f  repulsion  again  exceeds  the  one  of  attraction.  If,  beyond  this  point  b',  the 
force  of  repulsion  still  continues  the  greater,  the  curves  will  separate  more  and 
more,  and  will  approach  the  axis  A  X  without  ever  attaining  it,  so  that  at 
some  distance  A  xt,  infinitely  great  with  respect  to  the  one  A  a,  corresponding 
to  the  natural  state  of  equilibrium,  the  corresponding  ordinates  x,  z„  and  x,  yh 
will  be  infinitely  small  with  respect  to  the  one  a  b. 

Examining  now  what  takes  place  in  the  vicinity  of  the  point  b,  when  the 
natural  state  of  equilibrium  is  slightly  disturbed  by  any  extraneous  force,  it 
will  be  observed  that  when  the  force  acts  to  increase  the  primitive  distance 
A  a  between  the  molecuies,  so  as  to  make  it  A  a  -\-  a  x'  for  example,  then  the 
length  y'z'  =  x'z' —  x'y'  will  represent  the  value  of  this  force,  and  is  the 
intensity  of  the  resistance  offered  by  the  force  of  attraction  to  the  displace- 
ment a  x'  of  the  molecule.  In  like  manner,  it  will  appear  that  x  y  measures 
the  intensity  of  the  force  of  repulsion  to  an  extraneous  force  that  would  dis- 
place the  molecule  the  distance  a  x.  It  will  be  further  observed,  on  an 
examination  of  the  curves,  that  the  measure  of  the  intensity  of  the  resistance, 
offered  by  the  force  of  attraction  to  the  displacement  of  the  molecule,  will 
gradually  increase  with  the  displacement,  until  it  attains  a  maximum  state 
z'ti  y'",  corresponding  to  the  displacement  A  #"',  from  which  point  it  will 
decrease  to  the  point  b',  the  new  position  of  equilibrium  of  the  molecules. 

From  what  has  been  thus  far  stated,  a  clear  idea  may  be  formed  of  the 
elastic  resistance  offered  by  the  two  molecules  to  any  force  which  tends  to  dis- 
place them  from  their  state  of  natural  equilibrium,  and  the  law  between  this 
resistance  and  the  corresponding  displacement  within  the  range  of  elasticity. 
For  let  any  extraneous  force  be  applied  to  separate  or  bring  together  the  two 
molecules,  the  intensity  of  this  force  being  less  than  the  maximum  resistance 
ytit  z'",  its  effect  will  be  to  change  the  distance  between  the  molecules,  until 


378  APPENDIX. 

they  have  gained  a  new  position,  where  all  the  forces  will  be  again  in  equi. 
librium.  Let  this  position  be  the  one  corresponding  to  A  x'  for  example  ;.  nov; 
■o  long  as  the  extraneous  force  acts,  the  molecules  will  retain  their  respective 
positions  apart,  A  and  x' ;  if  the  extraneous  force  be  suddenly  withdrawn  the 
molecules  will  approach  each  other  to  regain  their  primitive  distance  apart 
A  a,  with  a  certain  velocity,  which  velocity,  or  rather  the  living  force  accu- 
mulated, will  cause  the  molecules  to  approach  nearer  to  each  other  than  the 
distance  A  a,  passing  which  the  force  of  repulsion  will  be  brought  into  play, 
and  bjr  its  resistance,  having  destroyed  the  living  force  gained,  will  cause  the 
molecules  to  recede  from  each  other,  creating  in  turn  a  certain  amount  ot 
living  force,  and  they  will  thus  continue  to  oscillate  between  their  primitive 
positions  until  they  are  finally  brought  to  rest  with  respect  to  each  other 
in  it  by  extraneous  resistances. 

If  a  tangent  be  drawn  to  each  of  the  curves  y,  y\  &c.,  and  z,  z',  &c,  at  their 
common  point  b,  these  tangents  will,  like  the  curves,  intersect  at  b,  and  will 
each  coincide  with  its  corresponding  curve,  for  a  greater  or  smaller  distance  on 
each  side  of  the  point  b.  Now,  if  any  distances  a  x',  a  x",  &c,  be  taken  on 
each  side  of  a,  and  be  considered  infinitely  small  with  respect  to  Aa,  the 
primitive  distance  apart  of  the  molecules,  the  portions  of  the  ordinates  to  the 
curves  as  n1  a',  n"  o",  at  these  points,  intercepted  between  the  tangents,  will 
be  equal  to  the  portions  of  the  same  ordinates  intercepted  between  the  curves, 
as  the  curves  and  their  tangents  are  taken  as  coinciding  along  the  portions 
corresponding  to  ax ,  a x",  &c.  The  intercepted  portions  of  the  ordinates, 
with  the  portions  of  the  tangents,  as  b  n',  b  n",  intercepted  between  them  and 
the  point  b,  will  form  similar  triangles,  from  which  is  readily  deduced  that  the 
portions  of  the  intercepted  ordinates  are  proportional  respectively  to  the  cor- 
responding distances  a  x',  a  x1',  &c. :  or,  in  other  words,  that  the  forces  with 
which  the  molecules  attract,  or  repel  each  other,  for  infinitely  small  displace- 
ments, as  compared  with  the  primitive  distance  of  natural  equilibrium,  are 

fl'  Qf  Jl'f  (jt 

proportional  to  the  displacements.  But  since  the  ratio = ,  between 

ax'  a  x 

the  portions  of  the  ordinates  intercepted  between  the  tangents  and  the  cor- 
responding displacement  is  constant,  it  may  be  taken  to  express  the  numerical 
value  of  the  resistance  offered  by  the  molecules,  in  their  position  of  natural 
equilibrium,  to  the  infinitely  small  displacements  in  which  the  tangents  coin- 
cide with  the  elements  of  the  curve ;  which  amounts  to  saying,  that,  for 
infinitely  small  displacements  of  the  molecules  of  a  body,  the  value  of  the 
elastic  force  remains  sensibly  constant. 

It  will  be  readily  seen,  from  an  examination  of  Fig.  A,  that,  in  proportion  as 
the  two  curves  approach  more  nearly  to  coincide  with  the  ordinate  a  b,  the 
angle  between  the  tangents  will  be  the  smaller,  and  the  distances  a  x',  a  x1' 
&c,  will  also  be  the  smaller,  as  compared  with  the  corresponding  parts  of  the 
ordinates  ri  o',  n"  o",  &c,  intercepted  between  the  tangents ;  and  the  elastie 
resistance,  or  rigidity   of  the  molecules,  measured  by  the    constant    ratio 

-,  will  be  the  greater. 

ax1 


A 


APPENDIX.  37C 

Relation  between  the  Eli  ".gation,  or  Compression,  and  the  Force  or  Strain, 
by  which  it  is  caused,  in  the  use  of  a  rod,  or  bar  of  a  given  cross  $e&ion,  the 
force  acting  in  the  direction  of  the  length  of  the  bar. 

Let  the  original  length  of  the  bar  be  represented  by  L;  the 
area  of  its  cross  section  by  A ;  by  W  the  force  acting  in  the 
direction  of  the  length  of  the  bar ;  which  force,  regarding  the  weight 
of  the  bar  as  inconsiderable  with  respect  to  W,  may  be  considered 
as  a  weight  suspended  from  the  lower  end  of  the  bar;  and  by  /  the 
elongation  of  L  due  to  W.  Now  whether  the  bar  be  supposed  to 
consist  of  as  many  parallel  fibres  as  there  are  equidistant  molecules 
in  the  section  A,  each  fibre  being  of  the.  length  L,  or  whether  it 
be  supposed  divided  into  a  number  of  infinitely  thin  slices  of  the 
same  thickness,  to  the  extremities  of  each  of  which  a  force  W  is  so 
applied  that  its  effort  will  be  uniformly  distributed  over  each  element 
of  the  area  A  of  the  slice ;  it  will  be  apparent,  from  a  moment 's 
consideration,  that  the  resistance  offered  by  the  bar  to  elongation 
will  be  independent  of  its  length,  and  proportional  to  the  number  of 
fibres,  or  to  the  area  A  ;  that  the  elongations  of  the  different  portions 
of  the  length  of  the  bar,  arising  from  the  action  of  W,  will  be 
directly  proportional  to  their  original  lengths,  so  that  the  total 
elongation  will  be  proportional  to  the  total  length  of  the  bar ;  and  the 
resistance  arising  from  elasticity  will  be  measured,  as  in  the  case  of  the  dis- 
placement of  two  molecules,  by  the  ratio  between  the  very  small  and  propor- 
tional displacement  of  two  molecules  of  the  same  fibre,  and  the  force  by  which 
this  displacement  is  produced. 

As  L  is  the  original  length,  and  I  the  total  elongation,  the  proportional 
elongation,  or  that  which  takes  place  for  each  foot,  or  other  unit  in  which  L  is 

expressed,  is  represented  by  the  fraction  —  =  X,  and  is  the  same  for  every 

fibre  of  the  bar.     The  measure  of  the  elastic  resistance  therefore  will  be 

W 

— .    Representing  by  E  the  measure  of  the  elastic  resistance  on  a  unit  of  the 

surface  A,  the  measure  of  the  total  resistance  on  this  surface  will  be  ExA. 
From  this  is  obtained  the  relation 

-=ExA;orW  =  EAA=EAi 
A  L 

from  which  W  or  I  may  be  found  when  the  other  is  known. 

By  making  A  equal  to  unity  of  area,  and  1=  L,  the  above  relation  becomes 
W  =  E.  In  other  words,  E  is  the  force  which  applied  to  a  bar,  the  area  of  the 
cross  sections  of  which  being  unity,  would  elongate  the  bar  a  quantity  equal  to 
its  original  length.  This  quantity  E  is  termed  by  writers  the  coefficient,  or 
the  modulus  of  elasticity. 

The  reasoning  here  used  for  the  circumstances  of  elongation  will  equaJy 
apply,  from  what  precedes,  to  the  case  of  the  shortening  of  a  bar  by  a  force  of 
compression. 


SSO  AFFElfDDC. 

To  find  the  relations  between  ike  elongation  and  strain  when  the  weight  oftkt 
bar  is  taken  into  consideration. 

Represent  by  L  the  original  length,  before  elongation,  of  a  bar  A  B 
(Fig.  B)  ;  by  x  the  length  A  C  of  any  portion  of  it,  estimated  from  A ;  by  dx 
an  element  of  the  part  x ;  by  W  the  weight  suspended  at  B ;  and  by  w  the 
unit  of  weight  of  the  material  of  the  bar. 

The  weight  of  the  portion  of  the  bar  B  C,  and  which  tends  to  elongate  the 
part  A  C  above  it,  will  be  expressed  by 

(L  —  x)  w A; 
the  total  force,  or  strain,  acting  at  the  point  C,  will  therefore  be  expressed  by 

W  +  (L  —  xl  wA; 
and  the  effect  of  this  strain  on  the  'vietipnt,  represented  in  length  by  dx 
will  be,  from  the  preceding  propor"'.K  i,  4>o  produw  an  elongation  expressed 
by 

W  -jr  'L  -  jc)  mL. 


EA 


ax; 


the  total  length  of  the  efomrcv'.  d  £  iiHr  elongation  will  therefore  be 

W  J.  (L--.-J  wA 
dx  r -g-s dx ; 

Integrating  this  exprvs«\or  betoken  the  limits  x  =  o  and  x  =  L,  there  obtains 

,       V,  h  +  lwUA 

L  + EA 

for  the  total  lerg*\  r*.  V  e  Lar  after  elongation. 

Relation*  b<-iwxv  ib".  F/rce,  or  Strain  applied  to  a  bar,  or  rod,  of  a  given  cross 
section,,  a~d  \hr  Irip'A,  df-c,  of  the  bar  when  rupture  ensues;  the  strain  being 
parallel  tr.  0  6  'lirct'on  of  the  length  of  the  bar. 

The  priaclpai  results  of  experiments  on  the  resistance  offered  by  materials 
jo  rupture  from  a  sirain  acting  either  to  compress,  or  tear  asunder  the  par- 
ticles, thus  calling  into  play  the  tenacity,  or  their  resistance  to  compression, 
Have  been  so  fully  given,  that  but  little  remains  to  be  said  here  farther  than  to 
fihow  the  effect  produced  by  the  weight  of  the  material  itself,  in  modifying  the 
strain  arising  from  any  external  force ;  also  the  manner  in  which  the  form  of 
the  bar  may  be  so  modified  as  to  make  it  most  suitable  to  resist  the  strain 
Arising  from  this  external  force  and  its  own  weight  combined. 

Suppose  a  bar  of  uniform  cross  section  thoughout  (Fig.  B),  the  area  of 
which  is  expressed  by  A,  and  its  length  A  B  by  L,  submi  .ted  to  a  strain 
*rising  from  a  weight  W,  suspended  from  B,  and  that  of  its  own  weight,  and 
Vet  w  represent  the  unit  of  weight  of  the  material  of  the  bar.  Representing  by 
R,  the  coefficient  of  rupture  of  the  given  material, — that  is  the  strain  that  would 
tear  asunder  a  bar  of  the  same  material,  the  area  of  the  cross  section  of  which 
is  unity, — then  the  resistance  offered  by  the  bar  A  B  will  be  expressed  by 

R  X  A. 
rhe  weight  of  the  bar  itself  will  be  expressed  by 

LA  to. 


381 


It  is  evident  that  the  greatest  strain  on  the  bar,  arising  from  the  combined 
action  of  W  and  its  own  weight,  will  be  at  the  point  A,  and  will  therefore  be 
expressed  by 

W+LAw; 
and  as  the  resistance  offered  by  the  tenacity  of  the  bar  must  be  equal  to  this 
strain,  there  obtains 

RA  =  W  +  LAw> 

to  express  the  required  relations. 

To  show  the  manner  in  which  the  form  of  a  bar  may  be  so 
modified  that  the  area  of  its  cross  section,  at  any  point,  shall 
be  just  sufficient  to  resist  the  strain  brought  upon  the 
material  at  that  point;  let  a  b  (Fig.  C)  be  such  a  bar  the 
length  of  which  is  expressed  by  L ;  let  any  portion  of  the 
length,  as  b  c,  be  expressed  by  * ;  let  w  represent  the  unit 
of  weight  of  the  material  of  the  bar ;  W  a  weight  suspended 
at  its  lower  end;  iet  the  cross  section  of  the  bar,  for 
example,  at  any  point  be  a  circle;  let  r  designate  the  radius 
as  c  d  of  the  cross  section  at  the  point  e ;  r"  the  radius  a  m 
of  the  top  section  ;  r  the  radius  b  n  of  the  bottom  section ; 
and  d  x  the  length  of  an  element  of  the  bar. 

The  area  of  the  cross  section  at  c  is  *  r3,  and  as  this 
area  is  supposed  to  vary  from  point  to  point  the  weight  of  a 
portion  of  the  bar  of  which  the  length  is  x  will  be  expressed 

by 

d  x; 


Fig.  C. 


•J 


and  the  strain  upon  the  section  at  c,  arising  from  this  weight  and  that  of  the 
suspended  weight  W,  will  be  expressed  by 


•/ 


rrr'dx  +  W; 


but  as  this  strain  must  be  just  equal  to  the  tenacity  of  the  bar  at  c,  and  i 
R  X  v  r"  represents  its  measure,  there  obtains 


'/ 


v  r»dx  +  W  =  R  X*r*; 


differentiating  this  expression,  there  results 

v>  it  r1  d x  =  2R  n  r  d  r 
hence 


which  integrated  gives 


d  r       w    , 


Log.  r  =  —  x  +  C, 


which  shows  that  the  curve  n  d  m,  cut  from  the  bar  by  a  plane  througn  iti 
centre,  is  a  logarithmic  curve. 

Without  passing  from  the  preceding  logarithmic  expression  to  the  equivalent 
numbers,  equation 


wivfdx  +  W^rXit*, 
can  be  placed  under  tt  e  form 

u>  fAdx  +  W  =  RA 

by  making  *•  r*  =  A;  differentiating  this  as  before,  there  result* 

«/  A  dx  =  R  d  A ; 

and 

d  A       u> 
— —  =  — <fa\ 
A        R 

Integrating  r  in  this,  between  the  limits  r'  and  r",  and  calling  the  respectrrt 

areas  at  these  points  A'  and  A" ;  and  also  integrating  x  between  the  limit* 

o  and  L,  there  obtains         % 

A"       w 

^A^R^ 

hence,  passing  to  the  equivalent  numbers, 

It 

A"  =  A'e  (B). 

but  as  the  strain  on  the  section  A'  is  W,  there  obtains 

W 
A'R  =  W,  andA'  =  — 

B 

Substituting  this  value  of  A',  in  the  preceding  equation  (B),  there  obtains 

to 

A»  =  f  efL    (C) 

for  the  value  of  the  area  at  the  upper  end  of  the  ban 
As  the  weight  of  anv  portion  of  the  bar  of  the  length  x  is  expressed  by 


J  A  dx        (D) 


w  I  Adx 


ard  the  value  for  any  variable  section,  at  the  distance  x  from  the  lower  end, 
is  found  by  substituting  A  for  A",  and  x  for  L  in  the  equation  (C)  just  pre- 
ceding, the  value  of  A  so  found  substituted  in  the  expression  (D)  gives 


dx 
R 


and  this  expression  integrated  between  the  limits  x  =  o  and  x  ■=  L,  becomes 

T  W  W   T 

—  e      .dx  =  W(e     — lj; 
which  is  the  value  of  the  weight  of  the  entire  bar. 


APPENDIX. 


383 


Relations  between  a  force  producing  the  rupture  of  a  solid  body  by  a  cross  strain 
on  its  fibres,  and  the  resistances  of  compression  and  extension  of  the  fibres  pro- 
duced by  the  action  of  the  force. 


The  effect  of  a  cross  strain  upon  the  fibres  of  a  solid  body,  as  a  bar,  or  rod, 
caused  by  the  action  of  a  force,  of  which  the  line  of  direction  is  either  perpen- 
dicular, or  oblique  to  that  of  the  fibres,  is  to  deflect  the  solid,  bringing  a  strain 
of  extension  upon  the  fibres  towards  the  convex  side,  and  one  of  compression 
on  those  towards  the  concave  side  of  the  solid.  Separating  the  fibres  which 
are  elongated  by  the  cross  strain  from  those  which  are  compressed,  it  is 
generally  assumed  that  there  exists  a  layer  of  fibres  which  is  not  affected  by 
the  cross  strain,  and  which,  on  that  account,  has  received  from  writers  on  this 
subject  the  name  of  the  neutral  line,  or  neutral  axis  of  the  solid.  It  is  also 
generally  assumed  that  when  the  deflection  is  inconsiderable,  the  elongations 
and  diminutions  in  length  of  the  extended  and  compressed  fibres  which  are  at 
equal  distances  on  each  side  of  the  neutral  axis  are  equal,  and  that  these 
changes  in  the  original  lengths  of  the  fibres  are  proportional  to  the  distances 
of  the  fibres  from  the  neutral  axis.  It  therefore  follows  from  what  precedes, 
that  so  long  as  the  elasticity  of  the  fibres  remains  unimpaired  under  the  action 
of  the  force,  the  resistances  offered  to  elongation,  or  compression,  will  be  pro- 
portional to  the  distances  of  the  fibros  from  the  neutral  axis.  In  the  state  of 
a  solid  immediately  bordering  on  rupture  from  the  effects  of  a  cross  strain,  it 
is  probable  that  the  elastic  limits  of  the  fibres  which  are  farthest  from  the 
neutral  line,  both  on  the  convex  and  concave  sides,  are  exceeded  before  they 
are  reached  by  fibres  lying  nearer  to  the  neutral  line,  and  that  the  resistances 
therefore  are  no  longer  strictly  proportional  to  the  distances  of  the  fibres  from 
the  neutral  axis.  But  as  the  hypothesis,  that  the  elasticity  of  the  fibres 
remains  unimpaired  up  to  the  moment  of  rupture,  approaches  more  nearly  the 
actual  state  of  the  question  than  any  other,  it  has  been  assumed  by  writers  on 
this  subject  as  the  basis  of  the  theory  from  which  the  formulas,  showing  the 
relations  between  the  forces,  are  obtained,  and  a  correction  for  the  imperfec- 
tion of  the  results  thus  arrived 
at  has  been  sought,  by  compar- 
ing them  with  those  obtained  by 
direct  experiment,  and,  by  means 
of  this  comparison,  deducing 
formulas  more  in  accordance 
with  the  actual  state  of  the  case, 
and  more  suitable  to  practical 
applications. 

Let  A  B  C  D  (Fig.  D)  repre- 
sent a  longitudinal  section  of  a 
bar  firmly  fastened  at  the  end  D, 
and  acted  upon  by  a  force  W,  at 
the  end  BC,  which  tends  to  deflect  the  bar.  Let  EF  be  the  position  of  the 
neutral  axis,  supposed  to  be  Ki.own,  and  OP  the  line  along  which  rupture  is 


1L._ 


a 


Pi«.  D. 


384 


about  to  take  place  from  the  effect  of  W.     Let  the 
area  comprised   within  the  curved   line  (Fig.    E)   bfl 
the  cross  section  of  the  bar  at  O  P.     Let  A  X  repie- 
sent  the  position  of  the  neutral  axis  on  this  cross 
section,  and  let  this  line,  with  the  one   A  Y   drawn 
perpendicular  to  it  at  the  point  A,  be  taken  as  the 
coordinate  axes  to  which  all  points  of  the  cross  section 
are  referred. 
Represent  by 
b,  the  breadth  of  the  cross  section  estimated  on  AX; 
d,  the  distance  from  AX  and  above  it  of  the  extreme  fibre  of  the  cross 

section,  or  the  one  which  is  most  elongated ; 
d,  the  distance  from  AX  of  the  one  most  compressed  ; 
x  and  y,  the  coordinates  of  any  fibre,  as  o  ; 
R,  the  coefficient  of  rupture. 
The  area  of  any  fibre  will  be  expressed  by  dx  X  dy ;  and  the  resistance 
which  the  extreme  fibre  from  AX  offers  at  the  instant  of  rupture  will  be 
expressed  by 

R  X  dx  dy. 

Now  as  the  resistances  offered  by  the  fibres  are  proportional  to  their  elonga- 
tions, or  compressions,  and  as  these  last  are  proportional  to  the  distances  of 
the  fibres  from  the  neutral  axis,  it  follows  that  d  being  the  distance  of  the 
extreme  fibre  from  AX,  and  y  that  of  any  other  fibre,  as  o,  from  the  same,  the 
resistance  offered  by  o  will  be  expressed  by 

-9-  R  X  dx  dv. 
d 

The  total  resistance  offered  by  all  the  fibres  will  therefore  be  expressed  by 
the  integral  of  this  last  expression,  or  by 


\fdXJ' 


ydy 


In  like  manner  the  total   resistance  offered  by  the  compressed  fibres  is 
expressed  by 


t/*/» 


dy 


Now  assuming,  at  the  instant  of  rupture,  that  the  deflection  of  the  solid  is 
inconsiderable,  and  that  the  force  \V  which  causes  it  is  perpendicular  to  the 
direction  of  the  fibres,  it  follows  that  the  conditions  of  equilibrium  require  that 
the  algebraic  sum  of  the  forces  in  the  direction  of  the  fibres  shall  be  equal  to 
zero,  and  that  the  sum  of  the  moments  of  all  the  forces  with  respect  to  the 
neutral  line  across  the  section  at  OP  shall  also  be  equal  to  zero. 

The  first  of  these  conditions  will  be  expressed  by 

—  j  dx  (  ydy——  J  dx  i  ydy=*o  (A) 


APPENDIX.  385 

as  the  resistance  to  elongation  offered  by  any  fibre  at  the  distance  y  from  the 

R  R 

neutral  line  is  —  dx  dy  yt  the  moment  of  this  resistance  will  be  —  dx  dy  y* 

u 

and  the  sum  of  the  moments  of  all  the  resistances  to  elongaUon  will  be 
expressed  by 

Tn  like  manner  the  sum  of  the  moments  of  the  resistances  to  compression 

Representing  by  z  the  perpendicular  from  OP  upon  the  line  of  direction  of 
W,  the  moment  of  VV  with  respect  to  the  neutral  line  across  OP  will  be 

Wz. 
To  express  therefore  the  second  condition  of  equilibrium  there  obtains 

y/^JV  dy  +  ~jj'dxJyi  dy — Wz = °-     (B) 

Representing  by  dA=.dxdy  the  section  of  a  fibre,  or  an  element  of  the 
cross  section  corresponding  to  it,  the  eq.  (A)  will  take  the  form 

d  d' 


_  /    yd  A  —  —  /     yd  A 

dJ  o  d'J  o 


which  expresses  the  condition  that  the  neutral  line  AX  (Fig.  E)  drawn  through 
the  suction  of  rupture  passes  through  its  centre  of  gravity.  When  the  neutral 
line  therefore  divides  the  section  of  rupture  symmetrically,  eq.  (B)  will  take 
the  form  t        ,, 


2—Jdxfyit!y=Wx        (C) 


The  sum  of  the  two  integrals  in  eq.  (B),  and  the  integral  which  forms  the 
first  member  of  eq.  (C),li;ive  received  the  name  of  the  moment  of  rupture.  The 
integration  being  affected  by  the  usual  rules  for  integrals  of  this  form,  the 
limits  of  x  being  taken  between  x  =  o  and  x  =  b,  and  y  being  either  constant, 
a  function  of  x,  depending  on  the  figure  of  the  section  of  rupture,  and 
its  limits  in  the  last  ease  being  y  =  o  and  y  =/(x),  the  resulting  equation  will 
express  the  relations  between  the  dimensions  of  the  solid  and  the  force  pro- 
ducing rapture,  when  the  value  of  R  has  been  suitably  ascertained  by 
experiment. 

Before  proceeding  farther  it  will  be  well  to  effect  the  integrations  for  the 
moment  of  rupture,  as  expressed  in  eq.  (C)  for  some  of 
the  more  usual  cases  where  the  cross  section  has  uni- 
form dimensions  throughout  the  entire  length  of  the 
solid. 

Taking  the  case  of  a  rectangular  cross  section  (Fig. 
F)  the  breadth  of  which  is  represented  by  b,  and  the 
depth  by  d  =  2  d',  the  expression  for  the  moment  of 


SS6 


APPES1UX. 


rupture  becomes 


2  *J*J  ?,k=  R 


6' 


In  the  case  of  a  circa ar  cross  section,  representing  by  r  the  radius  of  the 
circle,  the  expression  becomes 

2—      dxjfdy=2      -ps^-x1)    = 


i —  X  2  /  i 

r         J  > 


dx — 
3 


Rnr* 


Jb 

T 


In  the  case  of  a  tube  with  a  rectangular  cross  section  (Fig. 
G),  representing  by  b  and  d  the  breadth  and  depth  of  the  exte- 
rior rectangle,  and  by  b'  and  d'  the  like  parts  of  the  interior 
rectangle  which  forms  the  hollow  of  the  tube,  the  moment 
of  rupture  will  be 

bda  —  b'd» 


R 


6d 


Fig.  G. 


and  in  that  of  a  tube  with  a  circular  cross  section,  r  and  r' 
being  the  radii  of  the  exterior  and  interior  circles,  the 
expression  becomes 


R 


Ar 


In  the  case  of  a  uniform  cross  section  like  (Fig  H),  representing  by  b  the 
entire  breadth  of  the  solid  portion,  and  by  d  its  depth  :  by  b'  the  sum  of  the 
breadths  of  the  rectangular  voids  on  each  side,  and  by  d  their  depth,  the 
expression  becomes,  as  in  the  case  of  a  tube, 


R 


bd*  —  b'd'* 
~6d       ' 


The  foregoing  examples  will  serve  to  show  the  method 
of  obtaining  the  moment  of  rupture  in  all  like  cases  of 
solid  bars,  or  tubes  with  uniform  cross  sections  where  the 
neutral  line  divides  the  cross  section  symmetrically. 

Resuming  the  general  expression  (C),  which  shows 
the  relations  between  W  and  the  other  quantities,  and 
applying  it  to  the  case  of  a  beam  with  a  rectangular 
rross  section  of  uniform  dimensions,  the  length  of  which 
AB,  beyond  the  point  where  it  is  firmly  fastened,  is  repre- 
sented by  I,  the  expression  becomes 


i 

i* 

i 
i 

if 

i 

y 

r 

\> 

Fig.  H 


R^  =  w/; 


(D) 


APPENDIX. 


387 


supposing  the  rupture  to  take  place,  as  is  evident  from  the  eq.,  it  will  in  such 
a  case,  at  AD.     From  this  eq.  there  obtains 


W  =  R 


for  the  weight  which  the  beam  will  bear  at  the  moment  of  rupture. 

If  a  beam  is  laid  horizontally  on  two  props,  and  a  weight  is  placed  upon  it 
at  the  middle  point  between  the  props,  the  tendency  of  the  beam  to  rupture 
will  be  at  this  point ;  and  the  strain  upon  the  beam  from  W  will  evidently  bo 
the  same  as  if  the  beam  were  firmly  fixed  at  the  middle  point,  and  a  force 
equal  to  k  W  were  applied  at  the  point  where  the  beam  rests  upon  one  of  the 
props,  and  in  a  direction  contrary  to  that  of  W.  Representing  by  I  the  dis- 
tance between  the  props,  b  the  breadth,  and  d  the  depth  of  the  rectangular 
cross  section,  d  being  estimated  in  the  same  direction  as  W,  the  preceding  eq. 
<!))  will  take  the  form 


6 


jonsequently 


W 


,RT 


It  is  from  experiments  made  upon  beams  of  a  rectangular  section,  laid  hori- 
zontally upon  two  props,  and  broken  by  a  weight  placed  upon  them  at  the 
middle  point  between  the  props,  that  the  quantity  R  has  been  determined  for 
the  different  kinds  of  materials.  Having  determined  R  from  a  number  of 
such  experiments  the  value  of  W  can  be  determined  by  calculation  when  the 
quantities  b,  </,  and  I  are  known. 

If  instead  of  a  weight  acting  at  a  single  point,  a  beam  of  rectangular  cross 
section  is  strained  by  a  weight  uniformly  distributed  over  its  top  surface,  the 
conditions  of  equilibrium  from  which  the  relations  between  the  weight  and  the 
dimensions   of  the    beam   are    established  will    be    expressed    as    follows 
Representing  by  u>  the  weight  on  eauh  unit  of 
length  of  the  beam,  by  dz  (Fig.  I)  an  elementary 
length  of  the  solid  at  the  distance  z  from  the 
point  where   the  solid  is  firmly   fastened,  and 
where  the  rupture  will  take  place ;    then  the 
weight  distributed  over  the  element  dz  will  be 
wdz,  and  its  moment  with  respect  to  the  point 
of  rupture  will  be  wdz  x  z.     Calling  the  entire 
length  of  the  solid  I,  the  moment  of  the  entire 
weight    distributed    over  its    length    will    be 
expressed  by 

*l 
wdz  z; 


c 


3z 


I 


rig.  i. 


3 


Vwd* 


f 


883  APPENDIX. 

and  this,  from  the  conditions  of  equilibrium,  must  be  equal  to  the  moment  of 
rupture,  or 

>l 


But  wl  expresses  the  entire  weight  distributed  over  the  solid  ;  therefore,  front 
this  last  equation,  it  follows,  that  its  effect  upon  the  solid  is  the  same  as  if 
half  this  weight  were  suspended  from  the  end  of  the  solid. 

If  the  solid,  besides  supporting  a  weight  w  uniformly  distributed  over  each 
unit  of  its  length,  has  also  a  weight  W  suspended  at  its  end,  the  conditions  of 
equilibrium  will  be 

6  2 

Solids  of  equal  resistance. — This  term  is  applied  to  solids  in  which  the 
material  is  so  distributed,  with  respect  to  the  force,  that  the  strain  arising  from 
it  is  the  same  at  every  point  of  the  solid,  and  the  tendency  to  rupture  is  there- 
fore the  same  at  all  points.  In  a  beam  of  a  uniform  rectangular  cross  section, 
for  example,  placed  under  the  circumstances  supposed  in  the  preceding  cases, 
the  rupture  will  take  place  either  at  the  end  where  the  beam  is  firmly  fixed,  or 
at  the  middle  point  where  the  ends  rest  on  props.  As  these  are  the  weakest 
points,  there  will  be  an  excess  of  strength  at  all  other  points,  and  therefore  a 
waste  of  material.  These  considerations  have  led  to  an  examination  of  forms 
by  which  thus  waste  might  be  avoided.  The  change  of  form  of  the  solid  is 
usually  effected  by  varying  the  dimensions  of  its  longitudinal  section  made  by 
a  plane  through  its  axis,  the  plane  of  section  being  either  parallel  to  the  line 
of  direction  of  the  force,  or  perpendicular  to  it,  preserving  either  the  breadth 
or  depth  of  its  cross  section  the  same  at  all  points. 

Suppose  a  beam  (Fig.  K),  of  which  the 

upper  side  is  a  plane  surface,  placed  in  a 

horizontal  position,  firmly  fastened  at  one 

end,  and  strained  by  a  weight  W  at  the 

other;    it  is  required  to  determine  the 

form  of  its  longitudinal  section  by  a  plane 

through  its  axis  parallel  to  the  direction 

of  W,  the  breadth  of  the  cross  section 

being  the  same  throughout. 

ftg.  K.  Let  I  represent    the    length    of    the 

portion  of  the  beam  strained,  b  the  uniform 

breadth  of  its  eross  section,  d  its  depth  at  AD,  where  the  beam  is  fastened,  j 

the  deptn  of  the  cross  at  any  point,  the  distance  of  which  from  B,  where  W 

acts,  is  x. 

To  find  the  value  of  d  there  obtains  from  the  preceding  equations 

bd>  /6WJ 

RT=W/;andi  =  X/-^r; 


APPENDIX. 


389 


and  for  the  relations  between  y  and  x 

R  4-  =  w*  ;  hence  V  =  -«r-  —  -r ; 
6  K.0  Z 

A  relation  between  y  and  a;  which  expresses  that  the  longitudinal  section  of 

the  beam  must  be  terminated  at  the  bottom 
by  a  parabola,  the  vertex  of  which  is  at  the 
point  B.  From  the  conditions  of  the  pro- 
blem a  solid  so  shaped  will  offer  an  equal 
resistance  at  every  point  to  the  strain  arising 
from  the  action  of  W. 

Suppose,  as  in  the  last  case,  the  top  surface 
of  the  solid  plane  (Fig.  L),  its  breadth 
uniform,  and  that  the  strain  arises  from  a 

weight  w  uniformly  distributed  over  each  unit  in  length  of  the  top  surface. 
Adopting  the  same  notation  as  in  the  previous  case  there  obtains,  to  find  the 

value  of  d 


Fig.  L. 


6 


,  hence  d 


7         lZW 


and  to  obtain  the  relations  between  x  and  y, 


It 


W       wP  ,  dx 

—  sas  — ,  hence  w  =  — ; 
6  2  '  y         I 


a  relation  that  shows  that  the  lower  line  of  the  longitudinal  section  is  the 
right  line  BD, 

Suppose  a  solid  (Fig.  M)  with 
a  rectangular  cross  section  of 
uniform  breadth  throughout,  the 
lower  surface  being  plane,  laid 
horizontally  upon  supports  at  its 
extremities,  and  strained  by  a 
weight  placed  at  any  point  between  Fi    M 

the  points  of  support. 

Represent  by  b  the  uniform  breadth  of  the  cross  section,  by  d  its  depth  at 
the  point  where  W  acts,  I  half  the  horizontal  distance  between  the  supports, 
and  c  the  distance  between  the  point  D,  where  the  weight  acts,  and  the  point 
C,  the  middle  point  between  the  supports  A  and  B.  The  solid  will  evidently 
be  in  the  same  state  as  if  it  were  confined  at  the  point  D,  and  a  weight  equal 

to  W  —  were  applied  at  the  point  A,  and  in  a  contrary  direction  to  W.    Th« 

Am 

renditions  of  equilibrium  therefore  will  be 

6  21  v  T   J  21 

hence  the  depth  of  the  beam  at  the  point  A  is 


390 


AP^^^nxx. 


It  is  evident  from  the  preceding  that,  in  order  that  the  solid  shall  be  equally 
strong  throughout  in  this  case,  the  top  of  it  must  be  ft  rmed  of  the  portions 
of  two  parabolas,  the  vertices  of  which  are  at  A  and  B. 

Were  it  required  to  ascertain  the  form  of  the  longitudinal  section,  so  that 
the  solid  should  be  equally  strong  to  resist  the  action  of  the  weight  when 
placed  at  any  point  between  the  supports,  the  preceding  equation  of  equi- 
librium will  serve  to  establish  the  relation  between  the  parts.  For  represent- 
ing the  depth  of  the  solid  at  any  point  by  y,  and  by  x  the  corresponding 
distance  from  the  middle  points  between  the  props  to  the  point  D,  where  the 
weight  acts,  the  preceding  equation  will  take  the  form 

6 


■21 


which  is  the  equation  of  an  ellipse  of  which  the  vertices  are  at  the  points  A 
atid  B,  and  of  which  the  semi-conjugate  is  expressed  by 


/am 


Suppose  a  solid  (Fig.  N)  with  a  uniform 
thickness  d  in  the  line  of  direction  of  the 
weight,  but  of  variable  breadth,  subjected 
to  a  strain  arising  from  a  weight  uniformly 
distributed  along  the  centre  line  AB  of  the 
solid. 

Representing  by  y    any  ordinate  PM  of 
the  curve  which  bounds  the   top    surface 
pig  N-  of  the  solid,  and  by  x'  its  abscissa  PB  ;  by 

w  the  weight  borne  by  each  linear  unit  of 
AB ;  by  dx  any  elementary  portion  of  AB,  at  the  distance  x  from  B ;  then 
to  dx  will  be  the  weight  distributed  over  the  element  dx.  To  express  the 
conditions  of  equilibrium  between  the  moment  of  rupture  of  the  cross  section 
at  PM,  and  that  of  the  weight  of  any  portion  of  the  solid  of  the  length  x, 
■  there  obtains, 


R 


y-d* 


/•x1 
=    I  w  dx  (x1 — x)  =iwx  (2x' — x). 


Representing  by  I  =  AB  the  length  of  the  solid  from  DC  where  it  is  firmly 
fixed,  and  by  b  =  AC,  the  foregoing  expression  becomes 

R  —  =  twF ;  hence  b  =  . 

6  Her 

Since  for  the  cross  section  at  AC,  x  =  x'  =  I,  and  y>  =  b. 

The  preceding  expressions  show  that  the  curve  CMB  is  a  parabola,  of 
which  C  is  the  vertex,  and  the  line  CA  the  axis. 


APPENDIX.  391 

From  the  preceding  examples,  the  relations  between  the  moment  of  lapture 
and  the  strain  caused  by  a  force  acting  in  a  given  manner  on  a  solid  of  any 
form  of  cross  section,  whether  constant  or  variable,  may  be  established  when  the 
dimensions  of  the  cross  section  are  connected  by  any  geometrical  law.  In  the 
most  important  cases  in  structures,  these  relations  are  usually  expressed  in  the 
most  simple  terms  for  convenient  arithmetical  calculation,  the  constant, 
repivseuteui  by  R  in  the  preceding  equations,  being  determined  from  experi- 
ments made  on  the  solids  of  the  form  to  which  the  formulas  are  applicable. 
Examples  of  formulas  derived  in  this  manner,  and  adapted  to  arithmetical  cal- 
culation, are  given  in  Art.  320,  page  92,  and  at  the  end  of  Note  A  on  tubular 
bridges.  , 

A  huge  number  of  experiments  have  been  made  to  ascertain  the  value  of  R, 
by  submitting  solids  of  a  rectangular  cross  section  to  cross  strains  producing 
rupture.  As  might  have  been  anticipated,  the  values  of  R  so  determined  vary 
considerably.  The  mean  value  for  timber  is  usually  taken  at  12,000,  and  that 
for  cast  iron  at  40,000.  In  practical  applications,  the  value  for  timber  is 
reduced  to  the  one-tenth,  or  1200,  and  that  for  cast  iron  to  the  fourth,  or 
10,000,  of  that  determined  by  experiment,  when  the  formulas  are  used  to  fix 
ihe  strain  to  which  the  material  can  be  subjected  in  safety  in  the  ordinary  cases 
itf  structures. 

Manner  of  estimating  the  strains  on  frames  of  straight  beams  of  rectangular  cross 
section,  and  the  relations  between  the  strain  on  each  piece  and  its  dimensions. 

The  problems  presented  under  this  head  consist  of  two  parts:  the  first,  to 
And  the  directions  and  intensities  of  the  strains  on  the  component  parts  of  a 
frame,  composed  of  straight  beams  with  rectangular  cross  sections,  arising  from 
a  given  strain  acting  at  any  assumed  point  of  the  frame  ;  the  second,  so  to  pro- 
portion the  dimensions  of  each  part  that  it  shall  resist,  without  danger  of  rupture, 
the  strain  thrown  upon  it.  The  first  part  depends  for  its  solution  on  the  two 
fundamental  propositions  of  statics ;  the  parallellogram  of  forces  and  the  theo- 
rem of  moments.  The  second  is  but  an  application  of  the  preceding  formulas 
in  this  note. 

Solid  Built  Beams. — The  resistance  offered  by  this  class  of  beams  to  a  cross 
strain  will  depend  on  the  manner  in  which  the  pieces  are  placed  together,  and  the 
connexion  between  their  surfaces  of  contact.  In  beams  like  (Fig.  57),  p.  171, 
art.  502,  in  which  each  course  is  formed  of  two  or  more  beams  abutting  end  to 
end,  and  all  the  courses  are  firmly  secured  to  each  other,  so  that  the  surfaces  in 
contact  cannot  slide  on  each  other,  we  may  regard  the  strength  of  the  beam, 
supposing  the  courses  of  equal  thickness,  as  equal  to  that  of  a  solid  beam  having 
a  depth  equal  to  that  of  the  built  beam  diminished  by  the  thickness  of  one  course. 

Where  each  course  consists  of  a  single  beam,  and  the  whole  are  united  so  as 
to  prevent  sliding  (Figs.  59,  &c),  then  the  resistance  offered  may  be  regarded 
the  same  as  that  of  a  solid  beam  of  equal  depth. 

Adopting  the  same  notation,  as  in  the  preceding  part  of  the  note,  for  beams 
of  rectangular  cross  sectiot ,  the  formulas  for  a  solid  beam,  on  pp.  386-7-8,  may 


392  APPENDIX. 

be  used  in  these  cases  of  built  beams,  under  like  circumstances  of  the  position 
of  the  beam  and  the  point  of  application  and  direction  of  the  strain. 

The  formula  at  the  bottom  of  p.  389  is  applicable  to  this  class  when  sub- 
mitted to  a  strain  acting  at  any  point  between  the  centre  of  the  beam  and  ita 
points  of  support. 

In  all  such  cases,  however,  the  liability  of  the  beam  to  break  at  one  point  rathei 
than  another,  owing  to  the  manner  in  which  it  is  built,  must  be  carefully  con- 
sidered, and  a  suitable  modification  of  the  formula  be  made,  if  requisite.  The 
quantity  R  also,  which  enters  into  the  formulas,  and  which  for  solid  timber  it  ia 
stated  should  be  reduced  to  1200  lbs.  in  cases  of  practice,  may  be  further 
reduced  one-third  in  such  cases,  to  be  on  the  side  of  safety. 

Open  Built  Beams. — In  this  class  of  beams  (Fig.  63),  p.  172,  in  which  the 
breadth  and  depth  of  the  solid  parts  at  top  and  bottom  is  the  same,  the  relations 
between  the  weight  that  the  beam  will  bear  with  safety  applied  at  the  centre 
point  between  the  supports,  and  the  other  quantities  will  be 

bd*  —  b  d" 

W  =  iR'— 37— 

in  which  R'  is  the  reduced  value  of  R. 

Solid  Beams. — From  the  formula  (D),  p.  386,  by  substituting  R'  for  R,  there 
obtains 

Wl 

which  gives  the  relations  which  must  exist  between  the  weight  and  dimensions 
of  the  beam,  in  order  that  the  strain  on  the  unit  of  surface  of  the  fibres  shall  not 
exceed  the  quantity  R'.  If,  in  addition  to  the  strain  arising  from  W  applied  per- 
pendicularly to  the  axis  of  the  beam,  there  was  another  W  acting  parallel  to  the 
axis,  so  as  either  to  compress  or  extend  the  fibres,  this  would  cause  a  strain  on 
the  unit  of  surface  of  the  cross  section  expressed  by 

W 

bd' 

In  order,  therefore,  that  the  total  strain  arising  from  W  and  W,  applied  as  he»e 

supposed,  shall  not  exceed  that  which  the  unit  of  cross  section  can  be  submitted 

to  with  safety,  there  must  obtain 

Wl       W 

R=6T¥  +  Vd 

If  we  now  suppose  a  beam  (Fig.  O),  fast- 

i  ened  at  one  extremity,  to  be  subjected  to  a 

_^       strain  applied  at  the  other  obliquely  to  the 

Z3b''''  axis,  we  can  regard   the   oblique   strain  as 

replaced  by  its  two  components,  the  one  per- 

.... m.  pendicular  the  other  parallel  to  the  axis  of  the 

beam.     Representing  the  perpendicular  com* 

ponent  by  W.  and  the  parallel  one  by  W.  the 


Ffc.O. 


APPENDIX. 


| 

393 


Fig.  P. 


«am<?  relations  will  evidently  obtain  between  these  components  and  the  other 
quantities  as  in  the  preceding  case,  in  order  that  the  strain  on  the  unit  of  area  of 
the  cross  section  shall  not  exceed  R'. 

When  a  pressure,  or  a  force  of  extension  is  applied 
parallel  to  the  axis  of  a  beam  (Fig.  P.),  but  not  in  the 
same  line  with  it,  its  effects  will  be  to  compress,  or  extend 
the  beam  in  the  direction  of  the  axis,  and  to  produce  a  cross 
strain  on  the  fibres,  owing  to  the  tendency  to  bend  the 
beam. 

Supposing  a  vertical  beam  a  b,  fastened  at  the  point  A, 
and  a  weight  W  applied  at  the  extremity  of  another  beam 
B  c,  firmly  connected  with  the  first,  and  perpendicular  to 
its  axis.  The  compression  of  a  b,  in  the  direction  of  the 
axis  will  be  equal  to  the  entire  weight  W,  and  that  on  the 
unit  of  area  will  be 

W 

bd'' 

in  which  b  is  the  breadth,  and  d  the  thickness  of  a  b,  the  latter  estimated  in  the 
plane  through  the  axis  in  which  the  force  acts. 

Representing  by  h  the  horizontal  distance  b  c,  from  the  axis  at  which  W  acts, 
its  moment  will  be  equal  to  the  moment  of  rupture  of  the  fibres,  or 

Wh  =  $R'bdt 

In  order,  then,  that  the  strain  on  the  unit  of  area  shall  not  exceed  the  prescribed 
limit  there  obtains 

Wh       W 
R  =  6^~m  +  hi 


bd1 


bd 


Let  the  case  now  be  supposed  of  an  in- 
clined beam,  A  b,  which  rests  on  a  horizon- 
tal surface  a  c,  whilst  the  upper  end  lies 
against  a  vertical  surface  b  c,  the  beam 
having  a  weight  W,  suspended  from  its 
middle  point  o. 

Represent  by  I  the  length  a  b,  of  the 
beam,  and  by  a,  the  angle  between  its  axis, 
and  the  vertical  at  o. 

Leaving  out  of  consideration  the  fnction 
between  the  foot  of  the  beam  and  the 
horizontal  surface  on  which  it  rests,  it  is 
evident  that  the  reaction  of  the  vertical 
surface  against  the  upper  end  would  cause  the  beam  to  slide  along  a  c,  and 
that  to  prevent  this,  the  end,  a,  must  be  confined.  The  tendency  of  W,  there- 
fore, will  be  to  turn  the  beam  around  the  point  a,  and  this  will  be  counteracted 


Ftg.  a 


394  APPENDIX. 

by  the  reaction  of  b,  acting  horizontally.     Designating   Jus  foije  of  reaction  at 
b,  by  P,  there  obtains  from  the  theorem  of  moments, 

Pxad=Wxae; 
or,  substituting  for  a  d  and  a  e  their  respective  values,  I  cos.  a,  and  i  I  sin.  a 
P  =  £  W  tan.  a. 
By  confining  the  foot  of  the  beam,  an  equal  horizontal  force  is  called  into 
play  at  a,  whilst  the  total  weight,  W,  is  supported  at  the  same  point.     The  total 
pressure  at  a  will,  therefore,  be  the  resultant  of  these  two  forces,  which  are 
perpendicular  to  each  other,  and  will  be  expressed  by 


^W  +  k W'tan. *a  =  W    y/\  +  4tan.'a. 

Or  representing  by  A  b  and  A  c  the  directions  and  intensities  of  the  two  forces, 
W,  and  £  W  tan.  a,  the  diagonal,  A  d,  of  the  rectangle  will  represent  the  direction 
and  intensity  of  the  resultant. 

Having  thus  found  the  magnitude,  direction,  and  point  of  application  of  the 
two  forces,  there  is  next  to  be  considered  the  strains  to  which  they  subject  the 
fibres.  To  do  this,  the  beam  may  be  regarded  as  confined  at  the  point  o,  the 
lower  portion,  o  a,  being  acted  upon  by  the  two  forces,  A  b  and  A  c,  at  the  point 
A ;  the  upper  portion,  o  b,  by  a  horizontal  force  equal  to  a  b,  at  the  point  b 
Decomposing  the  force  A  b=£  W  tan.  a,  perpendicular  and  parallel  to  the  axis 
a  o,  the  components  will  be  represented  respectively  by 

£  W  tan.  a  cos.  a ;  and  £  W  tan.  a  sin.  a. 
The  components  of  W  in  the  same  directions  are 

W  sin.  a ;  and  W  cos.  a. 

A  will  be  observed  that  the  components  perpendicular  to  the  axis,  and  which 
Droduce  a  cross  strain,  act  in  opposite  directions  with  an  intensity  equal  to  their 
difference  expressed  by 

W  sin.  a — £  W  tan.  a  cos.  a=£  W  sin.  a. 

The  components  parallel  to  the  axis  act  in  the  same  direction,  compressing  ihe 
fibres  with  an  intensity  equal  to  their  sum,  and  expressed  by 

W  cos.  a  +  iW  tan.  a  sin.  a  =  W  cos.  a  (1  +  £  tan.  aa). 

Representing  by  b  the  breadth,  and  d  the  depth,  of  the  beam  estimated  in  the 
plane  of  the  forces,  there  obtains,  as  in  the  preceding  case,  to  represent  the  limit 
of  the  strain  on  the  unit  of  area 

p,  _     W  sin,  a  I      W  cos,  a.  (1  +  \  tan.*  a) 
K  ~  2       bd*       +  bd 

Considering  next  the  upper  half  o  b,  which  is  acted  on  only  by  the  horizontal 
force  at  b,  equal  to  £  W  tan.  a  ;  the  components  of  this  force,  perpendicular  and 
parallel  to  the  axis,  are 

i  W  tan.  a  cos.  a ;  and  i  W  tan.  a.  sin.  a. 


APPENDIX. 


393 


K 

3 

I 

A-         1 

„JSR— 

•1       lu 

■RP 

1 

1 

'"■/■■/ 

/'*'  S 

V. 

i 

Flg.R. 


The  perpendicular  component  producing  a  cross  strain,  and  the  parallel  one  a 
strain  of  compression.  To  express  the  limit  of  the  strain  on  the  unit  of  are* 
for  this  portion,  there  obtains 

_,       3  Wsin.  a  I          Wtan.  a.  sin.  a 
K  "1        bd*~  +  i bl 

To  prevent  the  flexure  of  a  horizonta 
beam,  as  a  b  (Fig.  R),  confined  at  one  end, 
and  sustaining  a  weight  at  the  other,  an 
inclined  strut  d  c  is  placed  beneath  it, 
abutting  against  some  fixed  point  as  d,  and 
against  the  beam  at  some  point  between  a 
and  b  ;  or  else  a  tie  is  used  leading  from  a 
point  c,  to  some  fixed  point,  as  e  above. 

Represent  by  I  the  distance  A  c,  by  V  that 
c  b  ;  by  a  the  angle  that  the  axis  of  the  strut 
makes  with  the  vertical. 

As  the  point  c  is  fixed,  the  weight  W,  act- 
ing at  b,  will  tend  to  turn  the  beam  around 
this  point,  and  this  will  call  into  play  a  ver- 
tical force  at  the  point  a  which,  from  the 
theorem  of  moments,  is  expressed  by 

the  fixed  point  c,  therefore,  sustains  a  vertical  effort  which  is  equal  to  the  sum 
of  this  force  and  W,  and  is  expressed  by 

I'  I  4-  V 

W+  W-i-  =  W±j-L- 

As  the  point  c  is  supported  by  the  strut,  the  vertical  force  at  c  may  be 
decomposed  into  components  in  the  direction  of  the  axes  of  the  strut  and  the 
beam.     Tliat  along  the  strut  will  be  expressed  by 

w  *±4 

/  cos.  a 
and  will  compress  it  in  the  direction  c  d.    The  one  along  the  axis  of  the  beam  is 

W-^±!taB.a. 

and  will  produce  a  "train  of  extension  on  the  part  a  c  of  the  beam.  This  por- 
tion A  c  is,  therefore,  subjected  to  a  cross  strain,  which  tends  to  rupture  it  at  c, 

from  the  force  N  —  it  ihe  point  a,  and  to  extension  from  the  force  acting  along 

its  axis..  .There  obtains,  therefore,  to  express  the  limit  of  the  strain  on  tno  unit 
of  area 


S96 


_,       6WT       W(l+l')imi.a 
it  =  ■  ,  ,.  -  + 


For  the  strut  there  obtains, 


bd* 


R'  = 


I.    bd 


W(l  +  V) 


1  cos.  a  b'  d' 
b'  and  d'  being  the  two  sides  of  its  rectangular  cross  section. 

If  the  beam  a  b  (Fig.  S)  is  supported  by  a  vortica. 
post  a  £  firmly  fastened  at  a,  the  strains  on  the  strut 
and  the  part  a  c  will  be  estimated  as  in  the  case 
just  examined.  As  to  the  strains  on  the  post,  it  will 
be  observed  that  the  force  acting  along  the  strut,  and 
which  is  represented  by 

w<±il, 
/  cos.  a 

_     LA>:,  will  be  transmitted  to  the  point  d.    This  will  be  equi- 

na?! valent  to  a  horizontal  force  represented  by 

111 


Fig.S. 

and  a  vertical  one  represented  by 


W  i^il  tan.  a; 


1 


W 


l+V 

1 


acting  at  c. 

From  the  connexion  formed  by  the  strut  between  the  horizontal  beam  and 
the  post,  the  effect  of  these  two  components  will  be  to  cause  a  pressure  on  the 
part  d  e  of  the  post,  which  will  be  equal  to  W;  and  to  produce  a  cross  strain  by 
the  horizontal  force,  the  moment  of  this  force  being  expressed  by 

=  W  (I  +  /'). 


W — : —  tan.  a  x 


1  "  tan.  a 

To  express,  therefore,  the  limit  of  the  strain  on  the  unit  of  area  of  this  part  D  K, 
there  obtains 

6  W  (I  +  Q       W 
bd*         +l~d' 

In  like  manner,  the  part  a  d  will  be  subjected  to  a  cross  strain  from  the  hori- 
zontal force  above-mentioned,  which  tends  to  cause  rupture  at  the  point  D,  and 
to  a  strain  of  extension,  acting  upwards  along  a  d,  from  a  vertisal  force  at  A 
expressed  by 

The  limit  of  the  strain,  therefore,  on  this  portion  will  be 

ewq  +  0      wr     ,vv 


APPENDIX.  397 

By  comparing  the  two  expressions  (X)  and  (Y),  it  will  be  seen  that  they  will 
be  equal  when  1=1',  in  which  case  the  port  will  have  an  equal  tendency  to 
rupture  at  d,  or  at  any  point  of  the  portion  d  e.  When  1  <  1'  then  the  tend- 
ency to  rupture  is  rather  in  the  part  d  e,  than  at  d. 

The  discussion  of  the  strains  on  (Figs.  R 
a        n       t*      n'       a'         an^  ^)  naturally  leads  to  a  consideration  of 
combinations  of  beams  represented  in  (Figs. 
R'  and  S'). 

In  combinations  of  this  kind,  the  simplest 
and  safest  plan    consists    in   supposing   the 
frame  resolved  into   parts,  such  that   either 
§j  alone  will  be  strong  enough  for  the  object 

^  in  view,  and  to  regard  their  combination  as 

affording  an  excess  of  strength  sufficient  to  in- 
sure perfect  safety  from  rupture.  In  (Fig.  R'), 
which  consists  of  a  horizontal  beam,  resting  on  the  points  of  support  a,  a', 
and  supported  at  the  intermediate  points  c,  c',  by  struts  d  c,  d'  c',  firmly  con- 
nected with  the  beam,  and  at  the  fixed  points,  d,  d'  ;  the  beam,  in  the  first  place, 
may  be  regarded  as  alone  supporting  the  weight,  W.  at  its  middle  point  b,  and 
the  area  of  its  cross  section  be  so  determined,  that  the  strain  on  the  unit  of  area 
Bhall  not  be  greater  than  the  limit  fixed  on.  Having  calculated  the  dimensions  of 
the  horizontal  beam  in  this  manner,  the  weight  may,  in  the  second  place,  be 
regarded  as  supported  by  the  portion  c  c'  of  the  beam,  and  the  two  struts  d  c 
and  d'  c',  in  which  case  the  strains  on  these  parts,  and  their  limits  on  the  unit  of 
area  of  their  cross  section  will  be  determined  by  considering,  that  at  the  points 
3  and  c'  a  vertical  force  £  W  is  acting,  the  components  of  which  in  the  direction 
C  D  and  c  b  are  respectively 

W 

a — - —  :  and  1 W  tan.  a. 
3  cos.  a '  2 

Representing  by  b'  and  d'  the  sides  of  the  cross  section  of  the  strut,  there 
obtains  to  express  the  limit  of  the  strain  for  it 

W 


R'  = 


2  cos.  a  b'  d" 


For  the  portion  of  the  beam  c  c',  it  may  be  regarded  as  confined  at  its  middle 
point,  where  W  acts,  and  subjected  to  the  two  forces  h  W,  and  i  W  tan.  a,  the 
first  producing  a  cross  strain,  and  the  second  one  of  compression  in  the  direction 
of  its  axis.  Representing  the  distance  b  c  therefore  by  t,  there  obtains  to 
express  the  limit 

6WT       jWtan.o. 
R  ~    2bd*     +         bd       *     (A> 

Representing  by  I  the  distance  a  c,  the:*  obtains  to  express  the  limit,  whe» 
the  beam  A  a'  alone  sustains  the  woight, 


APPENDIX. 


6W(1  +  Q 
K=        2bd»      * 


(Y\) 


How,  comparing  the  two  values  of  R'  in  the  expressions  (X')  and  (Y')»  it  «rlL 
oe  seen  that  the  combination  of  the  struts  and  tr^e  piece  cc'  alone  wiL  bi 
rtronger  than  the  beam  a  a'  alone,  when 

tan.  a  <  — r-. 
a 

In  like  manner  in  combinations  like  (Fig.  S') 
in  which  the  points  of  support  are  two  posts, 
a  e,  and  a'  e',  firmly  fastened  at  e  and  e',  we 
may  regard,  in  the  first  place,  the  beam  a  a 
and  the  two  posts  as  alone  sustaining  the  en 
tire  weight  W,  applied  at  b,  the  centre  point  of 
a  a',  and  ascertain  the  limits  of  the  strains  to 
which  these  pieces  can  be  respectively  sub- 
jected. Then  the  weight  may  be  considered  aa 
sustained  by  the  portion  c  c  alone,  the  two 
struts,  c  d,  and  c'  d',  and  the  posts.  Having 
ascertained,  as  in  (Fig.  S),  the  strains  on  thesw 
parts,  and  the  limits  to  which  they  can  be  sub- 
jected, if  it  be  found  that  either  of  these  combinations  alone  will  be  sufhVieudy 
strong,  then  will  the  entire  combination  have  an  excess  of  strength. 

As  the  beam  A  a'  is  firmly  connected  with  the  posts  at  a  and  a',  a  strain  of 
extension  will  be  thrown  on  the  portion  A  c,  from  the  pressure  at  d  acting 

W 


Fig.  S'. 


through  the  strut  c  d,  which  pressure  is  represented  by- 


This  again 


2  cos.  a 

may  be  regarded  as  equivalent  to  a  horizontal  and  a  vertical  force  respectively 

represented  by 

Wtan.a        ,  W 

>  and  — 

2  2 

Representing  by  ft,  and  h'  the  distance  e  d,  and  r  a,  the  horizontal  force  will 
cause  a  strain  at  a,  acting  in  the  direction  a  c  to  extend  this  part  represented  by 

W  ft' tan,  a 

2(h  +  hT 
and  one  at  the  fixed  point  e  represented  by 

W  ft  tan.  a 

2  (h  +  h')  " 
These  two  components  of  the  horizontal  force  at  d  will  cause  a  cross  strain  op 
3he  post.    We  may,  therefore,  regard  the  post  as  confined  at  the  point  d,  and 
*e  portions  on  each  side  of  d  subjected  to  a  coss  strain  from  the  horizontal 
components,  the  moments  of  which  components  will  be  the  same,  and  respect 
tvely  represented  by 


APPENDIX.  8S: 

W  h  h'  tan.  a 

and  to  a  strain  in  the  direction  of  the  axis  represented  by  .—.    The  limit  there 
fore,  of  the  strain  to  which  the.  post  can  be  subjected  will  be 

R'  =     W  6WU'  tan,  a 

2b' d'       2  b'  d'% (h  +  h') 
in  which  b'  and  d'  are  the  sides  of  the  rectangular  cross  section  of  the  post. 

Straining  and  Tie  Beams.— -If  the  struts  c  d,  c'  d'  (Figs.  R',  S'),  instead 
of  being  connected  with  the  beam  a  a',  abutted  against  a  straining  beam  placet 
beneath  the  portion  c  c',  as  shown  by  the  dotted  line,  then  the  horizontal  strain 
on  the  beam  a  a',  from  the  action  of  the  struts,  would  be  borne  by  the  strain- 
ing  beam  alone.  This  portion  of  the  frame,  composed  of  this  beam  and  the  por- 
tion c  c'  of  the  beam  a  a',  may  be  regarded  as  a  single  beam  submitted  to 
a  cross  strain  from  the  weight  W,  and  to  a  strain  of  compression  on  the  strain- 
ing beams  alone  from  the  horizontal  component  of  the  pressure  at  the  point  c  or 
c',  and  the  relations  between  the  limits  of  the  strain  on  the  unit  of  area  and  the 
dimensions  of  the  beams  be  found  as  in  the  previous  cases. 

If  the  struts,  instead  of  abutting  at  d  and  d'  against  the  wall,  or  posts,  were 
confined  by  a  tie  beam  shown  by  the  dotted  lines  d  d',  this  beam  would  relieve 
ihese  points  from  the  horizontal  pressure,  and  would  itself  be  subjected  to  a 
strain  of  extension  equal  in  amount  to  the  horizontal  strain  on  the  straining 
beam. 

In  the  preceding  cases,  the  frame  has  been  considered  as  subjected  to  the 
action  of  a  strain  acting  at  one  point  alone ;  if,  in  addition  to  this,  the  beam  a  a' 
was  subjected  to  a  strain  uniformly  distributed  along  it,  the  vertical  pressures  at 
c  c'  and  a  a'  would  have  to  be  increased.  Representing  by  w  the  weight  uni- 
formly distributed  on  the  unit  of  length  of  a  a',  the  total  weight  on  the  portion 
a  c  will  be  represented  by  to  I,  and  that  on  the  portion  b  c  by  w  I' ;  the  verti- 
cal pressure  at  c  arising  from  these  two  will,  therefore,  be  represented  by 

i  w  I  +  w  V, 
and  this  must  be  added  to  the  vertical  component  of  W  at  the  same  point,  in  all 
the  preceding  formulas,  to  make  them  applicable  to  this  case. 

Combinations,  like  (Fig.  T),  com- 
posed of  two  inclined  pieces,  a  c 
and  b  c,  abutting  against  each  other 
at  a,  and  confined  at  b  and  c,  either 
by  being  inserted  into  a  horizontal 
tie  beam,  or  abutting  against  fixed 
points,  may  be  arranged  either  for 
sustaining  a  vertical  strain  at  the 
point  a,  as  that  arising  from  a 
*fc*T  weight  suspended  at  a;   or,  aa  is 


400  APPENDIX. 

he  case  of  the  rafter?  of  a  roof  truss,  to  sustain  a  pressure  uniformly  distri 
buted  over  the  two  inclined  pieces. 

Representing  by  p  and  q,  the  angles  between  the  inclined  pieces,  and  tha 
vertical  through  a,  the  components  of  W,  in  the  directions  A  B  and  A  c,  wiL 
be  respectively  represented  by 

sin.  9       _  ^hxp 

sin.  (p  +  q)'  sin.  (p  +  ?)' 

and  the  horizontal  pressure  at  a,  which  is  the  same  as  the  horizontal  strain  at 
the  points  b  and  c,  is  represented  by 

sin.  p  sin.  q^ 
sin.  (p  +  q) 

Each  of  tnese  beams  may,  therefore,  be  regarded  as  confined  at  their  lower 
points,  and  subjected  at  their  upper  ones  to  the  strains  in  the  direction  of  their 
axes  just  determined ;  and  from  these  the  relations  between  the  limits  of  the 
strain  on  the  unit  of  area  and  the  dimensions  of  the  beams  can  be  determined  as 
in  the  preceding  examples. 

When  the  angle  p  is  equal  to  q,  the  strains  in  the  direction  of  each  beam  will 
be  expressed  by 

W 


and  the  horizontal  strain  by 


2  cos.  p' 


i  W  tan.  p. 


Were  the  beams  in  this  last  case,  instead  of  supporting  a  vertical  pressure  at 
tne  point  a,  each  subjected  to  a  vertical  strain  applied  at  any  point  between  a 
and  the  foot  of  each  beam,  it  is  obvious  that  each  would  be  in  the  same  condi- 
tion as  the  one  (Fig.  Q),  and  that  the  horizontal  strains  at  the  points  a,  b,  and 
c,  those  in  the  direction  of  the  axes  of  each  beam,  and  the  total  pressure  at  b 
and  c,  would  be  found  as  in  (Fig.  Q)  ;  and  from  these  the  relations  between  the 
dimensions  of  the  beams  and  the  limit  of  the  strain  on  the  unit  of  area  be  in  like 
manner  determined. 

The  applications  of  the  preceding  problems  to  the  analogous  cases  of  frame?! 
composed  of  rigid  materials,  as  timber  and  iron,  will  be  readily  seen  on  a  com- 
parison of  the  forms  in  Art.  507,  and  the  following,  with  those  which  have  just 
been  the  subjects  of  examination.  The  main  difficulty  in  each  case  will  be  in 
determining  those  strains  both  in  intensity  and  direction,  which  arise  from  the 
reciprocal  action  of  the  component  parts  of  the  frame,  and  this  will  be  most 
easily  overcome  by  supposing  the  parts  pressed  against  removed,  and  their  resist- 
ance replaced  by  forces  acting  in  the  same  directions  as  the  resistances ;  as,  fs 
example,  in  the  case  of  (Fig.  Q),  where  a  horizontal  force  at  b  might  be  made 
to  replace  the  resistance  of  the  wall  against  which  the  beam  rests,  &c. 

In  all  cases,  moreover,  whare  a  strict  equilibrium  does  no*  obtain,  either  ^ 
the  strains  being  transmitted  to  fixed  points,  or  by  equal  counteracting  forces, 
the  stability  of  the  framing  will  depend  on  the  joints,  or  connexions  of  the 
beams;  and  wherever  there  is  a  tendency  to  the  rotation  of  any  one  of  the 
beams  around  a  joint,  it  must  be  prevented  by  the  introduction  of  a  strut  or  tie, 
so  placed  as  to  hold  the  beam,  liable  to  this  movement,  in  a  fixed  position . 


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PROF.     D      H.     MAHAN,     LL.D 


An  Elementary  Course  of  Civil  Engineering, 

For  the  Use  of  the  Cadets  of  the  United  States  Military  Academy.     1  vol.  8vo. 
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appear 

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v. 

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*  Altogether  tne  vade  mecum  of  tnls  Country." — IIoJtTicoLTUWBT. 


THE  NEW  REVISED  EDITION 

of 

DO  W  KING'S 

FRUITS    AND    FRUIT    TREES 

OP 

AMERICA, 

OK  TIM  CULTURE,  PROPAGATION,  AND  MANAGEMENT  IN  THE  GARDEN  AND  ORCHARD  OF  FRUIT  TREE* 

GENERALLY,    WITH    DESCRIPTIONS    OF    ALL    THE   FINEST    VARIETIES    OF    FRUIT, 

NATIVE  AND    FOREIGN,    CULTIVATED    IN   THIS   COUNTRY, 

Br  A.  J.  DOWNING. 

Revised,  Corrected,  and  greatly  Enlarged  by 

CHARLES    DOWNING. 

One  thick  vol.  12mo.,  of  779  pages.     Xumerous  cuts.     Full  cloth.     $1  75. 


This  edition  is  intended  to  furnish  a  complete  manual  of  American  Pomology,  and 
contains  as  far  as  can  at  present  be  ascertained  the  character  and  habits  of  all  the 
important  fruits  of  every  district  of  the  country. 

It  has  been  prepared  by  Chas.  Downing,  whose  personal  knowledge  of  most  of 
the  different  varieties  which  are  grown  in  various  sections  of  our  country,  has  pecu- 
liarly fitted  him  for  giving  a  reliable  estimate  of  the  value  of  the  different  varieties, 
as  well  as  accurate  descriptions.  The  errors  of  the  former  editions  have  been  cor- 
rected, and  although  nothing  has  been  omitted,  the  additions  have  been  so  great  as  to 
give  it  almost  the  character  of  a  new  work;  for  instance,  the  last  former  edition 
contained  of  apples,  about  170  kinds,  the  present  over  6u0;  of  pears  230,  the  present 
560  ;  and  of  other  fruits  many  kinds  have  been  added. 

The  descriptions,  although  concise,  are  remarkable  for  their  systematic  accuracy, 
but  the  feature  which  gives  greatest  value  to  the  book  in  addition  to  its  comprehen- 
siveness is  the  classification  by  which  the  relative  merits  of  all  the  fruits  are  so 
clearly  indicated  as  to  be  seen  at  a  glance 

Notices  of  tlie  Press. 

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can Agriculturist. 

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JOHN  WILEY,  5G  WALKER  ST.,  N.  Y. 

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Downing's  and  other  Agricultural  Works 

PUBLISHED    Bt 

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DOWNING,  A  J.     THE  FRUITS  AND  FRUIT  TREES  OF  AMERICA, 

Or,  the  Culture,  Propagation,  and  Management  In  the  Garden  and  Orchard  of  Fruit 
Trees  generally;  with  descriptions  of  all  the  tim-^t  varieties  of  fruit,  native  and  foreign. 

cultivated  In  this  country.  Now  edition,  thoroughly  revised,  with  very  large  additions, 
especially  in  apples  and  pears.  Edited  by  Charles  Downing,  Esq.,  brother  of  tho  late 
A.  J.  Downing.     One  vol.  12ino.,  containing  over  750  pfl{rnn     $1  50. 

••  No  man  who  has  a  plot  of  50  feet  square  should  be  without  this  book;  while  to  the 
owner  of  acres  it  is  beyond  all  price." — Newburgh  Gazette. 

"  This  book  is,  therefore,  in  our  opinion,  the  very  best  work  on  Fruits  that  we  have."— 
American  Agriculturist. 

"  We  hail  the  present  work  as  the  best  American  Fruit  Book  extant."—  Ohio  Culti 
tutor. 

DOWNING,  A.  J.         COTTAGE    RESIDENCES: 

A  Series  of  Designs  for  Rural  Cottages  and  Cottage  Villas,  and  their  Gardens  and  Grounds, 
adapted  to  North  America.  Illustrated  bv  numerous  engravings.  Third  edition.  8vo. 
Cloth.    $2  oa 

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ed city  and  the  man  of  taste  who  retires  with  a  full  purse,  to  embody  his  own  ideas  of 
rural  home.'' 

in. 
DOWNING,  A.  J.        LINDLEY'S    HORTICULTURE. 
With  additions.    One  vol.  12mo.     $1  25. 

DOWNING,  A.  J.        LOUDON'S  GARDENING  FOR   LADIES, 
And  Companion  to  the  Flower  Garden.    By  Mrs.  Loudon.    12tno.     Cloth.    $1  25. 

DOWNING,  A.  J.  WIGHT  WICK'S  HINTS  TO  YOUNG  ARCHITECTS, 
Calculated  to  facilitate  their  practical  operation;  with  additional  Notes  and  Hints  to 
Persons  about  Building  in  the  Country.    Svo.     Cloth.     $1  50. 

PARSONS   ON 'THE   ROSE. 
The  Rose — Its  History,  Poetry,  Culture,   and  Classification.    With  two  large  colored 
plates,  and  other  engravings.     In  one  vol.  12mo.     New  edition,  with  additions.     Cloth. 

*1    O.I. 

"This  elegant  volume,  devoted  to  a  subject  of  universal  attractiveness,  and  exhausting 
most  of  tho  learning  which  applies  to  it,  deserves  a  wide  popularity." 

vi  r. 
KEMP   ON   LANDSCAPE    GARDENING. 

How  to  Lay  Out  a  Garden.  Intended  as  a  general  Guide  in  choosing,  formlt  «•,  or  Im- 
proving an  estate  (from  a  quarter  of  an  acre,  to  a  hundred  acre*  in  extent),  with  leference 
to  both  design  ami  execution.  By  Edward  Kemp,  Landscape  Gardener,  Birkenhead 
Park.  Greatly  enlarged,  and  Illustrated  with  numerous  plans,  sections,  and  sketches  of 
gardens  and  garden  objects.     1  vol.     12mo.     Cloth.     Gilt.     |2  00. 

"This  is  just  the  book  that  thousands  want." — X.  Y.  Observer. 

"It  should  he  in  the  hands  of  every  "tie  who  makes  even  the  slightest  pretensions  to 
Gardening." — I'fila.  City  Item. 

VIII. 

CLAUSSEN.     Till';    FLAX   MOVEMENT. 
Its  Importance  and  Advantages  ;  with  Directions  for  the  Preparation  of  Flax  Cotton,  and 
the  Cultivation  of  Flax.     By  the  Chevalier  Claussen.     12mo.     12  cents. 

LIEBIGk     PRINCIPLES   OF   AGRICULTURAL   CHEMISTRY 
With  special  reference  to  the  late  researches  made  in  England.    1  vol.  12mo.    Cloth.   50c. 

*t*  Copies  will  be  mailed  to  unv  addrett  and  prepaid,  on  the  receipt  of  the pric* 
Ctws  a:id  SocieUe*  icill  be  supplied  with  the  tcorisjor pretniunix,  at  a  discount. 


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